Firefox -- for the adherence to standards and because it is faster. I also use the following extensions:
Colorzila
Web Developer
IE Tab
Gmail Manager
PDF Download
Foxmarks
I still use IE (usually in a firefox tab) for OWA and to check web sites that I'm developing. And usually I have to spend 2 to 3 times as long to resolve IE issues with website code then it does with firefox. And I usually have the most problems with IE not handling CSS or javascript correctly. And IE6 and IE7 treat the errors differently.