Multiple Firefox Versions on Same PC

Yes. Download and install them in seperate directories. Then, launch each one individually with the -p flag to set up different profiles for each version (or at least one for testing). Then, after you have two seperate profiles, create an icon for each on your desktop. Right click on the icon and select properties.

In the 'target' field, add the following flags.

c:\Programs\Firefox 3.0\firefox.exe -p Profile1 -no-remote
c:\Programs\Firefox 3.5\firefox.exe -p Profile2

This is assuming you will be using Firefox 3.5 as your main browser and 3.0 for testing. If you want it the other way around switch the -no-remote tag. This allows you to run multiple versions of Firefox side by side. For more information refer to the Mozillazine page on command line arguments.

You could also try Utilu which automates this process, and also installs the Web developer toolbar and Firebug on each version. Very handy for a test machine.

You can use the portable versions of firefox (3.5, older versions). You can install as many versions of firefox side-by-side as you want, but you can only run one version at any time.