You could use a portable version for one of them. You could alternately set up a separate profile for one of them. * http://kb.mozillazine.org/Files_created_on_startup * http://kb.mozillazine.org/Profile_Manager * http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

You can install multiple Firefox versions via a custom install in different installation folders and only use the older Firefox version when you really need it. 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 * http://releases.mozilla.org/pub/mozilla.org/firefox/releases/ Make sure not to start Firefox after the installation has finished.<br /> Create a new profile exclusively for each Firefox version.<br /> Create a desktop shortcut with -P "profile" appended to the target to launch each Firefox version with its own 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 * http://kb.mozillazine.org/Bypassing_the_Profile_Manager Use the -no-remote command line switch to open another Firefox instance with its own profile and to run different Firefox instances simultaneously. * http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

ENGLISH-ENGLISH!!!!!!!