It works as well/poorly/adequately as Thunderbird does, as one would expect. As you can see from the website, they claim different compiler and other optimizations result in a more stable product.
If you use the 64-bit version of FossaMail AND Lightning, at the moment you need to turn off automatic updates since the updater will install the 32-bit version of Mozilla Lightning rather than the 64-bit Pale Moon Lightning, breaking stuff. I don't know if this issue has been fixed.