Seems just fine for me. Never had issues with latency or bulkiness except some earlier versions (5, or 6 iirc that had some issues with some NVidia drivers crashing at some cases).
Xmarks, Firebug, FlagFox and IE Tabs are just full of win plugins. Never bothered with heavy plugins installations or Personas.
For me Firefox it's just fine, and matured much over the years. Opinions ofc differ but it's true that many peepz are turning to Chrome these days.
