However the version 4 brings numerous changes such as a completely new GUI, better HTML and Flash rendering support, etc... , if you love to be on the side of the true open source community, then as a web browser you can't miss Firefox4 nonetheless. If you want to install or upgrade to the latest stable release of Firefox4 in Ubuntu, then use the following method.
As usual, open the terminal (Applications -> Terminal) then issue the following commands.
sudo add-apt-repository ppa:mozillateam/firefox-stable
sudo apt-get update && sudo apt-get upgrade