It's because since FF3.5 they only made the browsing experience worse. To the point I hate updating Firefox, because it there's a chance it will do something silly I'll have to fix.

But all the other browsers are even worse. I'd really like to move to qutebrowser, but it doesn't have good enough adblocking. I'd give Zen a try, too, but it's not in the repositories of my Linux machine and I'm never installing a browser that is not in the repositories.