I was a Firefox user back when it was called Phoenix and gave it up around 3.x due to UI responsiveness issues, bloat, etc.
In the mean-time I've become a Mac user as well and love Safari on OS X.
I recently tried to use Firefox on Windows again. It has proven to be infuriating. The UI locks up if any particular tab is busy. IE9/10/11 is far more responsive and works with pretty much every site I care about. So I've ditched firefox again, and use it only for downloadhelper when I want to rip videos.