Here is the simple way to use the new firefox 3.5 on ubuntu
- Goto http://en-us.www.mozilla.com/en-US/firefox/upgrade.html
- Click on the Free Download button
- It will download the file firefox-3.5.tar.bz2 to your machine
- Extract this using a archive manager to a folder of your choice
- Goto the extracted folder and double click on the firefox shell script in the extracted folder
- You are now running firefox 3.5