FossaMail is a Thunderbird-based mail, news and chat client from the developers of the Pale Moon browser.
The program features Pale Moon-like compiler optimizations "geared towards an as stable and smooth operation of the client as possible", and also uses the Pale Moon browser core as its back end.
FossaMail looks and feels just like Thunderbird, and it should run all Thunderbird (v24/25b era) add-ons and plugins.
FossaMail stores its data in a custom location (%APPDATA%\FossaMail), allowing you to install and run FossaMail and Thunderbird on the same system.
There's also an optional calendar, Lightning, available from the developer's site.
What's New? (Changelog)
This is a security and stability release, updating the client's back-end to Pale Moon 25-LTS 25.9.5.
This incorporates all relevant core security/stability updates from the latest Pale Moon version.
This version also changes the default setting for the "condstore" (IMAP conditional store) to have it disabled, since there are some implementation issues on servers and clients that cause discrepancies which prevent proper flagging of newly arrived mail. Condstore also does not offer a way to track removed messages, which is a serious problem and has caused confusion for a number of users.
FossaMail is a straightforward email client with plenty of worthwhile features. We're unsure if it's significantly better than Thunderbird, but you can install and run the programs together, so it's easy enough to compare the two.