I use Seamonkey because it has sane UI interface elements like "buttons+text" that are actually larger than 15 pixels so that I can click on them without having to exactly aim my mouse on a tiny little button. My guess is that every Firefox developer still runs a 2007 Asus eeebook with a 7" screen. I got more pixels than I care to count, so I just want a UI that scales in proportion with the screen.

Just one example why Seamonkey is important (for me at least).