SeaMonkey 2.53.5

You MUST absolutely do a full backup of your profile before upgrading from a version earlier than SeaMonkey 2.53.1. This version of SeaMonkey makes changes to your profile that can't be reverted in cases where you need to go back to the older version.

Automatic upgrades from previous versions are currently disabled for this release. Please download the full installer from the downloads section and install SeaMonkey 2.53.5 manually over the previous version.

The Mozilla backend code SeaMonkey is based on, now usually incorporates breaking changes in every release. This is especially true for the Mozilla Gecko add-ons code and the JavaScript interpreter. Consequently, some older extensions may no longer function properly. If you experience any problems please refer to the troubleshooting section first. Especially try to disable extensions first and check if this makes a difference.
Important only if you upgrade from 2.49.5 or an earlier version: Due to changes in the user profile, if you have set a master password, you will have to remove it before upgrading to SeaMonkey 2.53.5. After a successful upgrade, don't forget to manually remove the files key3.db and cert8.db in your profile afterwards, as they include credentials and passwords no longer protected by a master password. You can find the path to your profile folder entering the special about:support URL in a SeaMonkey browser window.
You can remove your master password by providing an empty password in the "Change Master Password" dialog. Do not use the "Reset Password" function. You will lose all stored passwords this way.
If you have a SeaMonkey 2.53.1 or 2.53.2 language pack installed you need to remove it before upgrading to this version. These language pack versions were marked to be compatible with all Seamonkey 2.53.x releases and cause breakage when used with a different version. You can then install one of the provided 2.53.5 language packs from the download section after installation of the base product.

For SeaMonkey 2.53.5, the following extensions are included.

  • ChatZilla
  • DOM Inspector
  • Lightning
Beta versions also contain the following extension.
  • SeaMonkey Debug and QA UI

These release notes describe system requirements, installation instructions, and known issues for SeaMonkey 2.53.5 up to/including SeaMonkey 2.53.5. These notes are updated when we receive feedback, so please check back for new information.

What's New in SeaMonkey 2.53.5

SeaMonkey 2.53.5 contains (among other changes) the following major changes relative to SeaMonkey 2.53.4:

  • Provide WebP support bug 1653869.
  • Add as a search engine available to all locales bug 1655283.
  • Added av1 support.
  • Included latest version of freetype2.
  • Added support for the resizeObserver web API.
  • Support for rust 1.47.0 on Linux and macOS platforms.
  • Dropped support for use of system sqlite.
  • Various security and general platform fixes.

SeaMonkey 2.53.5 contains (among other changes) the following major changes relative to SeaMonkey 2.49.5:

  • The Bookmarks Manager has switched its name to Library, and now also includes the History list. When invoking History, the Library will be shown with the History list selected. The extensive modifications were needed because of Mozilla Gecko platform API changes.
  • Download Manager has been migrated to a new API. Although it looks pretty much the same as before, the search option is missing and some other minor details work differently. The previous downloads history is removed during the upgrade.
  • The layout panel was added to the CSS Grid tools.
  • TLS 1.3 is the default SSL version now.
  • The only NPAPI plugin which will work with SeaMonkey 2.53.5 is Flash. Support for other NPAPI plugins like Java and Silverlight has been removed. For displaying pdf files in the browser you can use pdf.js-seamonkey from Isaac Schemm.
  • SeaMonkey now uses a new api for formatting regional data like time and date. Default is to use the application locale of the current SeaMonkey build. If you use a language pack or a different OS formatting this is usually not desired. You can change the formatting from the application locale to the regional settings locale (OS) in the preferences dialog under "Appearance".

SeaMonkey 2.53.5 uses the same backend as Firefox and contains the relevant Firefox 60.8 security fixes.

SeaMonkey 2.53.5 shares most parts of the mail and news code with Thunderbird. Please read the Thunderbird 60.0 release notes for specific changes and security fixes in this release.

Additional important security fixes up to Current Firefox 78.4 ESR and a few enhancements have been backported. We will continue to enhance SeaMonkey security in subsequent 2.53.x beta and release versions as fast as we are able to.

SeaMonkey-specific changes

  • SeaMonkey now uses gtk3 on Linux. If you experience a problem because of this please file a bug and link it to bug 1367257. Please try another OS theme first. Some of them are buggy and cause problems with SeaMonkey, Thunderbird and Firefox.


The SeaMonkey project provides official versions and language packs of SeaMonkey 2.53.5 for Windows, Linux and macOS in US English and a small variety of other languages. Community volunteers are also contributing builds for other platforms. You can download SeaMonkey 2.53.5 here.

System Requirements, Installation and Uninstallation

Before installing, make sure your computer meets the system requirements.
Please note that the new Windows x64 version will only support Windows 7 x64 and up. You need to uninstall a previous Windows x86 version if you want to use it. Installing the x64 over the old x86 is not supported and does not remove the old version which might cause problems later.
SeaMonkey 2.53.5 will no longer offer to migrate your data from SeaMonkey 1.x or Mozilla 1.x at the first start after installation (bug 689437). In order to upgrade from such an old version, install the last SeaMonkey 2.0 release first, do the one-time profile upgrade, uninstall SeaMonkey 2.0 and then install your target release (e.g. this one).
If you are on an older release we suggest upgrading to SeaMonkey 2.49.5 first.
You can find more details about installation, profile data and uninstallation in our install and uninstall document.

Extensions (Add-ons) and Themes

Extensions and Themes for SeaMonkey 2.53.5 can be installed from the SeaMonkey Add-Ons website or the "Get Add-Ons" section in the Add-ons Manager.

Extensions with binary components such as Enigmail only work for the specific platform they are released (e.g. win extension for Windows, linux64 for Linux 64 bits). Besides, matching version of extensions for SeaMonkey may not be the latest available of those versions. Therefore, they need to be adjusted for each new SeaMonkey release, which also means that an old version of such an add-on can break SeaMonkey if you disabled compatibility checking (which e.g. happens when you install the Add-on Compatibility Reporter add-on). Please, note the following warnings and advice regarding extensions:

  • For Enigmail, please refer to the Enigmail page.
    The latest supported version is 2.0.9 Enigmail but there have been some problems reported against it.
  • Adblock Plus 2.9.1 may cause huge memory and CPU consumption and is not recommended or officially supported in this SeaMonkey version. Consider switching to uBlock.
  • Lightbird 0.7.1. is not compatible with this version. The message preview pane will not update itself correctly.
  • The latest 1.16.x working version of uBlock can be found here. Please install the latest legacy version.
  • The latest working version of NoScript Classic 5.x can be found here.
  • Dictionaries in WebExtension formats do not work, even though they appear to install correctly. Support for them will be added at a later date. Please install the latest classic version from the dictionary history.
  • Lightweight themes are no longer provided on the SeaMonkey Add-ons site.
  • Full theme add-ons may need changes because of user interface and internal changes. If you find any problem with themes, contact the theme author. Before reporting a problem with the user interface please make sure to recreate it with either the Classic or Modern theme.
  • Because of security backports which removed obsolete code, Session Manager add-on version or above is required to be compatible with current and future 2.53 releases.

Known Issues

This list covers some of the known problems with SeaMonkey 2.53.5. Please read this before reporting any new bugs.
  • Clearing the browser history during shutdown does currently not work. Please use "Tools->Clear Private Data" as a workaround. The problem is tracked in bug 1621445
  • Opening single new tabs from the Library always loads them in the foreground. The problem is tracked in bug 1619108
  • SeaMonkey will not start under Linux if you downloaded the wrong package.
    We are providing x86 and x64 versions of SeaMonkey for Linux. For newer distributions you probably need the Linux x64 version.
  • Lightning or one of the distributed extensions is not showing up or is disabled under Add-ons.
    If the extension is disabled and can not be enabled deinstall it. If it does not show up after restart do the following:
    • Enter "about:config" in the location bar.
    • Enter "extensions.installedDistroAddon" in the search field.
    • Reset the key of the extension not showing up using the popup menu with the mouse. If in doubt just reset all keys, but this might bring back previously uninstalled distributed Add-ons.
    • Restart SeaMonkey.
  • Unable to connect to secure sites and a certificate error is shown.
    In 99% of all cases this issue is caused by the same problem. Your antivirus software or custom firewall acts as a man in the middle and intercepts secure https traffic. In order to intercept and scan this encrypted network traffic, it normally uses a vendor specific certificate which is not installed by default in SeaMonkey by the third party software installer. If you want to keep this feature working you need to install one or more certificate(s) from the antivirus software in SeaMonkey. You need to ask your antivirus software vendor where they are located. Consult the SeaMonkey online help to find out how you can import and manage certificates.
  • If you experience a hang and/or get a message box warning that a script is busy when opening the Data Manager try deleting the webappsstore.* files from your profile folder when SeaMonkey is not running. These files contain data stored by websites and will be recreated during the next start. If you are unsure if the information is needed please back up the files before deleting them. The problem is tracked in bug 1305624 but so far no coding error has been found. This problem has been reported frequently when trying to manage passwords. But the problem will generally happen just after you open the Data Manager, because this component is also used for managing cookies, site preferences permissions and other settings.
  • If you are running Windows 7 "Send to > Mail recipient" might not work. More information can be found in bug 1356902. As a workaround install the "Microsoft Visual Studio 2015 to 2019" redistributable runtime library. For x86 install the VS2019 x86 runtime.
    If you are using the x64 build install the VS2019 x64 runtime. SeaMonkey was built using the latest VS2019 compiler so make sure to upgrade any older VS2017 runtime.
  • If you experience a delay showing the first page after start, please delete the Safebrowsing folder in your profile. Bad Safebrowsing data may delay showing the first page for up to 30 seconds. The problem is tracked in bug 1432274. This can occur even if you are not using Safebrowsing because the lists are always fetched.
  • SeaMonkey does not currently support the WebExtensions add-on api. Some popular add-ons like NoScript and uBlock Origin are no longer shown because of this on the SeaMonkey add-ons website. You can usually get them from the manufacturers site. WebExtensions support is a planned feature for 2.57. For dictionaries please install the latest non webext version.
  • The SSL 3.0 encryption protocol is no longer supported due to security concerns (bug 1106470). Legacy websites may still rely on it and will not work with the newer TLS 1.x protocols. Contact the website administrator to update their server to resolve the issue at the source.
  • The Remember Passwords add-on breaks password handling. Please disable or uninstall this add-on using the Add-ons Manager.
  • You cannot set up a new Sync account or pair a device anymore (bug 998807).
  • Data loss warning: If you use a profile with this or any later version and then try to go back to SeaMonkey 2.49.5 or earlier, SeaMonkey will rename your bookmarks and history file to places.sqlite.corrupt and create a new places.sqlite file, effectively resetting your browsing history. This is because the internal format changed and the upgrade process only works in one direction. Additionally, running MailNews with local folders and POP3 mail filters may produce summary files that are not correctly read by previous versions of SeaMonkey. If you decide to go back to a previous version of SeaMonkey, you should do it from a profile backup only.
  • After creating a news account through clicking a news URL, the MailNews account wizard may be broken (the account type will always be News). To work around the problem, set the mail.server.serverX.valid pref (where X is the internal number of the corresponding news account) to true in about:config, or delete the invalid news account (bug 521861).
  • WebRTC has been disabled by default. It needs an add-on to enable it per site. See the webrtc-permissions-ui-toggle add-on.
  • Web pages may appear differently in the MailNews feed reader than in the browser (bug 662907). Workaround: Add a "noscript {display:none}" rule to your userContent.css (note that the Stylish add-on cannot be used here since it does not integrate with the MailNews feed reader).
  • Lightning keyboard shortcuts for accessing the calendar tab from within the MailNews window or from the main menu (Ctrl+Shift+C/D) do not work. (bug 514512).
  • If you try to start SeaMonkey using a locked profile, it will crash (see bug 573369).
  • Windows:
    • You might not be able to install SeaMonkey in Windows 10 if you have turned on the Windows Defender Smartscreen feature. Your SeaMonkey download will be blocked in the case. To overcome this go to "Windows Security->App & browser control" in the new Windows Settings and set the "Check apps and files" setting to Warn from Block.
    • Antivirus scanners may report that some parts of SeaMonkey (e.g. the file freebl3.dll) are suspicious. If you downloaded SeaMonkey from one of the official download sites, this is a false alarm. You might experience problems with secure websites when this happens. To fix the issue, instruct your antivirus software to ignore these files (and move them out of quarantine) and/or switch to another antivirus software and reinstall SeaMonkey.
  • Mac:
    • You might not be able to install SeaMonkey under recent macOS versions. In this case please read Open a Mac app from an unidentified developer for a solution.
    • When hardware acceleration is off, background colors in the UI (toolbar, status bar etc.) might not update correctly when you switch focus from/to a window. Users running Macs with certain older hardware will experience this even if they haven't turned off hardware acceleration in Preferences (Appearance/Content) (bug 626096).


  • Some questions are asked more frequently than others. Have a look at the SeaMonkey FAQ. Maybe your particular problem has already been solved.
  • Some antivirus software keeps detecting our installers as containing a "Trojan" or "Downloader". This is a false positive because of some wrong detection mechanism in the antivirus software. The code in question is actually from 7-Zip and decompresses the installer so that its contents can run and install SeaMonkey. That technology is widely used and probably can be used to compress bad as well as good software, but the 7-Zip code itself is perfectly safe. Mozilla scans all our files routinely when putting them on the FTP server and we assure you that our installers are as clean as the official Firefox downloads that are scanned by the same mechanisms.
  • For security, parts of bug 1035091 were ported in bug 1411708. This might cause problems with add-ons like Stylish which use moz-document. Set the preference "layout.css.moz-document.content.enabled" to "true" to overcome this but be aware that this disables the added security fix.
  • Poorly designed or incompatible extensions can cause problems with SeaMonkey, including make it crash, slow down page display, etc. If you encounter strange problems relating to parts of SeaMonkey no longer working, the browser not starting, windows with strange or distorted appearance, degraded performance, etc, you may be suffering from Extension or Theme trouble. Restart SeaMonkey in Safe Mode. On Windows, start using the "Safe Mode" shortcut created in your Start menu or by running seamonkey.exe -safe-mode. On Linux, start with /path-to-seamonkey/seamonkey -safe-mode or first change to the SeaMonkey installation directory and start with ./seamonkey -safe-mode, and on macOS, run:

    cd /Applications/
    ./seamonkey-bin -safe-mode

    When started in Safe Mode all extensions are disabled and the Default theme is used. Disable the Extension/Theme that is causing trouble and then start normally.
  • If you uninstall an extension that is installed with your user profile (i.e. you installed it from a Web page) and then wish to install it for all user profiles using the -install-global-extension command line flag, you must restart SeaMonkey once to cleanse the profile extensions datasource of traces of that extension before installing with the switch. If you do not do this you may end up with a jammed entry in the Extensions list and will be unable to install the extension globally.
  • If you encounter strange problems relating to bookmarks, downloads, window placement, toolbars, history, or other settings, it is recommended that you try creating a new profile and attempting to reproduce the problem before filing bugs. Create a new profile by running SeaMonkey with the -P command line argument, choose the "Manage Profiles" button and then choose "Create Profile...". Migrate your settings files (Bookmarks, Saved Passwords, etc) over one by one, checking each time to see if the problems resurface. If you do find a particular profile data file is causing a problem, file a bug and attach the file.

Frequently Asked Questions

What can I do to help?
We need help from developers and the testing community to provide as much feedback as possible to make SeaMonkey even better. Please read these notes and the bug filing instructions before reporting any bugs to Bugzilla.
Why haven't you responded to the mail I sent you?
Use the newsgroup. The SeaMonkey team reads it regularly, and your email may have gotten lost.
Where can I get extensions and themes (add-ons)?
Extensions and Themes can be downloaded from SeaMonkey Add-ons.
Who makes SeaMonkey?
Lots of people. See Help->About SeaMonkey, contributors for a list of some of the people who have contributed to Mozilla and SeaMonkey.
Where's the SeaMonkey 2.53.5 source code?
A tarball of the SeaMonkey 2.53.5 source code is available at the download page. The latest development code can be obtained from the source repository. SeaMonkey-specific source is in the "suite" directory. Please follow the build instructions. We will provide source code for 2.53.1 and future releases on gitlab.
Can I remove the tool- and menubar grippies?
You can remove the tool- and menubar grippies by setting the preference "browser.toolbars.grippyhidden" to "true".

Other Resources and Links

The following resources contain useful information about SeaMonkey 2.53.5: