Now that Firefox 4 final is out, I wanted to switch to using it rather than version 3.x on my Ubuntu 10.10 systems. You can download a Linux version of Firefox 4.0 from the Mozilla site, but it's 32-bit only. While it will run, the problem I found is that all of my plugins (libflashplayer, etc) stopped working. The solution is to install the 64-bit build of Firefox 4, which is available on a launchpad PPA. Thus:
sudo apt-add-repository ppa:mozillateam/firefox-stable sudo apt-get update sudo apt-get upgrade
That gave me a new, working FireFox 4.0. Most of my extensions worked OK, with the exception of del.icio.us (fix here), and All-in-one sidebar (now uninstalled). And, weirdly, the language packs aren't compatible between 3.x and 4.0, but I assume that will get sorted in time.