You can read full realese note @ Firefox 9 Release Note
Download and install Firefox 9 in ubuntu 11.10
sudo sudo apt-add-repository ppa:mozillateam/firefox-stable sudo apt-get update sudo apt-get install firefox
If PPA Firefox stable already exists in your ubuntu system. to upgrade firefox 8 to firefox 9 you just type in terminal.
sudo apt-get firefox
Just it, have fun with firefox 9 on ubuntu