From the forum:
In addition, a full range of language packs are now offered.This major update to the mail client involves the following:
- Change of the core scripting and rendering back-end to Pale Moon 25.1! FossaMail is now completely based on Pale Moon, using it as the "engine" to drive the mail/news client.
This basically fast-forwards the mail client a year in terms of features and improvements on the back-end, including improved secure connections to servers and the likes.
This also means FossaMail can no longer be considered a "re-build of Mozilla Thunderbird" and should be considered a true fork from this point forward.
Updated branding and a few aesthetic things.
FossaMail now has a repository on GitHub - considering release of the source code at this point (with a lot more than just branding and configuration changes) is required as per the MPL Open Source license.
It is also confirmed that FossaMail will continue to be compatible with Thunderbird. For those of us old codgers who do not like the new Mozilla UI, this is a great alternative.
Chris