SeaMonkey Nightly Builds
Because of Mozilla platform API and source code removals Nightly builds are currently not provided. This will not change anytime soon. We have allocated all available developer resources to fix up the next planned major release (expected to be 2.57).
The 2.57 defects in comm-esr60 are tracked in Bug 1433370.
Additional Nightly defects in comm-central are tracked in Bug 1452448.
We are in the process of creating a new infrastructure to switch to and, once that is ready, will look to provide alpha and beta builds. In the meantime you can test unofficial builds provided on Bill Gianopoulos home page hosting unofficial builds. As with all unofficial releases be careful and have a full backup of your profile ready. Downgrading to prior versions was never and will not be supported.