For Ubuntu Users:
Firefox 9 will be available as a security update via the official repositories for Ubuntu 11.10 Oneiric Ocelot and 11.04 Natty Narwhal users, but if you don’t want to wait, you can already install it by using the Ubuntu Mozilla Security PPA:
sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa
sudo apt-get update
sudo apt-get install firefox
* For Ubuntu Lucid and Maverick, Firefox 9 should be available in the Firefox Stable Channel PPA soon: “ppa:mozillateam/firefox-stable” (in fact, the packages have already been built successfully, but publishing has been disabled for the PPA for now).