The rapid release schedule has helped Firefox a lot, plus the stability with extensions even with the nightly releases are major factor. I've been using the x64 minefield < nightly releases exclusively for their 64bit support in Windows and Chrome soured that real web experience. It doesn't exist!!!
Two of my fav extensions (Adblock + NoScript) in Chrome aren't equally efficient as in Firefox, the flakiness of flash with Chrome is another can of worms entirely.