First, download the tarball from the official website, then uncompress it using:
tar -xjf firefox-3.5.tar.bz2
Then, place the firefox directory somewhere inside your home directory, say, ~/apps/, where ~ is your home directory. Next, make a shortcut to the firefox executable - right click on the desktop, click on Create Launcher, and at the Command field enter the command below:
Replace /path/to/firefox/ and USER. For example, if your username is ubuntu and you copied the firefox directory inside ~/apps/ (where ~ is your home directory), this command should look like:
The -P switch will start Firefox 3.5 using the profile manager, so from here all you need to do is to create a new profile for your Firefox 3.5:
Click on Create Profile...
Choose the name of the profile
Additionally, you can specify the --no-remote switch (firefox --no-remote -P) so you can run both your currently 3.0.11 and 3.5 instances at the same time. The --no-remote switch doesn't connect Firefox to a currently running instance.
Firefox 3.5 running in Ubuntu 9.04 Jaunty
Updated: July 1, 2009