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.