Once upon a time, there were some funny GPL issues that influenced Firefox's place in Ubuntu.
I remember using swiftfox, swiftweasel and iceweasel, instead. I see that the Icecat and Gnuzilla are now available as alternatives. It might be fun to play with them at some point.
At one point, the latest releases were coming out on launchpad before they showed up in the standard repositories. If that's available maybe I'll try that again.