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).
Firefox on Windows respects the DPI settings (unlike Chrome!). So if you enable the relevant options in Windows' display settings, everything in the Firefox UI and webpages, including icons gets scaled up appropriately.