I LOVE Firefox and my extensions. My nephew recommended it to me last Aug and I've been very happy with it. The rest of my family refuse to use it. They like IE7. UGH! IE freezes up on me.

I would prefer Opera if it came with the variety of extensions Firefox has. But it doesn't so Firefox is my browser of choice.

Firefox--all the way ! ! Even with the new TABBED IE7,I can't imagine how frustrating it would be without an extension similar to TabMixPlus to enable bouncing around among the tabs the way I like to.
I have found, though, that installing extensions slowly,deliberately, one at a time,makes for a more stable browser. And thinning the herd -uninstalling- any that you find are not serving your purposes keeps it lean.