Firefox with occasional use of Chromium (chromium is the open source version of chrome, I use it mainly because it's in the ubuntu repos).
I prefer firefox for a few reasons though:
1. Chrome's extensions aren't as good as firefox's. You can duplicate most of the important ones, but the still don't work as well (NotScript, Adblock, etc.)
2. It really irks me that Google seems to have some of their pages hard coded into the browser with special treatment. Go to the Chrome Web App Store, and you'll notice that none of your extensions are working. AFAIK, Google has that page hard baked into the browser to disable extensions for 'security'. I could understand this, but there is no switch to disable it.
3. The EULA you sign with chrome is annoying, although not the end of the world I guess. Still, you can install Chromium without signing a EULA, but you don't get nifty features that are closed source (pdf reader for example).