How to run multiple versions of Firefox simultaneously
Install two versions of Firefox. Version A and B in separate folders.
Launch Firefox profile managment by running "firefox -P" in Run window (Window + R).
Create a new profile. It’s sensible to name the profile the version of Firefox that you wish to run "firefox_A".
Click on "Choose folder" to set where the profile will be stored. In this folder is where profile information such as add-ons will be stored. Again it is sensible to use the target Firefox version number.
Create a shortcut to Version A of Firefox.
Right click and select the properties of the shortcut.
In the target field specify which Firefox profile should be used by appending at the end of the field "-P firefox_A -no-remote". Include a space between the closing quote. e.g. "C:Program FilesMozilla Firefox 3.6firefox.exe" -P firefox_A -no-remote.