Do a custom install and install each version in its own program folder to use multiple Firefox versions. * https://support.mozilla.com/kb/Custom+installation+of+Firefox+on+Windows Create a new profile exclusively for each Firefox version. Create a desktop shortcut with -P "profile" appended to the target to launch each Firefox version with its own profile. See these mozillaZine KB articles for information: * http://kb.mozillazine.org/Testing_pre-release_versions * http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows * http://kb.mozillazine.org/Shortcut_to_a_specific_profile * http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox * http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile