Having second thoughts because you think your favourite Firefox extensions will not work with Firefox 3.6? I use about 40 add-ons and they all work, all you need to do is tweak FF a little (read about: making extensions compatible with any Firefox version).
Note: Tab Mix Plus users, install THIS version.
1. Add the Ubuntu Mozilla Daily PPA:
-Ubuntu Karmic:
sudo add-apt-repository ppa:ubuntu-mozilla-daily/ppa
-Older Ubuntu versions:
sudo sh -c "echo 'deb http://ppa.launchpad.net/ubuntu-mozilla-daily/ppa/ubuntu UBUNTU_VERSION main' >> /etc/apt/sources.list"
sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 247510BE
In the first command above, replace UBUNTU_VERSION in the line above with jaunty, intrepid or hardy, depending on your Ubuntu version.
Or use the automatic Launchpad PPA script to add the GPG key.
2. Install Firefox 3.6:
sudo apt-get update && sudo apt-get install firefox-3.6
Update 22 January, 2010: It seems there's an error with the latest Firefox 3.6 and AppArmor (permission issues). See a fix, HERE.