Firefox 9.0 stable was released two days ago.has updated the packages to the latest but only for Ubuntu 10.04 & 10.10, and other Ubuntu editions can try Firefox 9 by downloading code from Mozilla.org, or waiting for the repository to update.
Now,has updated the packages of latest Firefox 9 for Ubuntu 11.10 & 11.04.
If you can’t wait to upgrade your Firefox to latest, open up a terminal window and run following commands to add the ppa:
sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa
sudo apt-get update
sudo apt-get install firefox