I have been using Seamonkey since when it was called Netscape, but there are so many sites that do not work with it that I been forced to switch to Firefox. I still use Seamonkey for my mail though, I prefer it to Thunderbird.
What are the main differences between Thunderbird and SeaMonkey’s e-mail client?
What matters to me are certain small differences in the UI.
Doesn’t it use the same engine as modern versions of Firefox?
It does use the same engine from Firefox, although not always the latest version. This project strives to backport security updates from newer Firefox versions whenever they can, sometimes being able to upgrade the base Firefox version, while keeping the legacy XUL based add-on support. This is actually one thing I like wtih Seamonkey, alongside its HTML editor (that which could see better support for CSS and Javascript/ECMAScript stuff) and GUI based image blocking.
No, it's stuck with the engine at Firefox 68 (from 2019) with backported security fixes. A lot of the XUL removal stuff has meant that SeaMonkey basically needs to rewrite their UI to update to current versions
Oh wow, that’s quite old; I hope they can find a way to backport newer versions at some point.