I'd recommend checking out WaterFox. It's what I switched to when I finally got sick & tired of Mozilla's shit.

i really feel like trying this out as a quasi-firefox user, but i've really started to love and appreciate Zen for its UI :( wonder if there's a Waterfox X Zen alternative.

EDIT: whoops, should've scrolled down a bit on the website, looks like Waterfox has vertical tabs as well. damn, probably going to try to migrate to it sometime soon...

EDIT2: of course supports firefox extensions as well, perfect.