Just make sure that you install each version in a separate program folder (do a custom install if necessary) and that you use a separate profile folder for each version. I don't know if Waterfox is using the same file profiles.ini to register profiles as Firefox does, but in case that it does then make sure to create a new profile. * 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