Firefox and Chrome have been fighting for top dog for me for a while. Sometimes Firefox has its bugs so I'm required to fallback to Chrome, then Chrome develops a problem and I'm forced to use Firefox again until it starts having issues.
At the moment, however, I'm using Chromium 17. I also have Firefox Nightly installed for when the day does come that there is a problem.
I enjoy both the browsers, they both have their strong points, but they have way too much in common, and the add-on library is similar.
I do, however, use Firefox to watch Ustream and Livestream.com, because Adblock plus can block their ads.