Ubuntu 10.04 and 10.10 users who want to upgrade to Firefox 4 can do so with the Firefox Stable PPA.
You can install or upgrade to Firefox 4 by entering 2 commands in the Terminal under Applications > Accessories > Terminal. Enter the following two commands in the Terminal one line at a time, entering your password when prompted:
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update && sudo apt-get upgrade