I've been using the Firefox 4 beta for a while now and like it a lot--it's fast and a great improvement on Fox 3xxx although I won't say it's better than Chromium. It's now out of beta, so if you want to add the ppa to your system, fire up a terminal and type in the following:
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update
sudo apt-get install firefox ubufox
and you'll be in business.