Yes, you can use Firefox 3.5.x on Windows 7 If you need older versions to access websites then it is better to use a portable Firefox version or install that older version alongside via a custom install in a separate program folder. In the latter case you need to create a separate profile folder exclusively for that version as well. The portable version has its own profile folder, so using that version is easier. * http://portableapps.com/apps/internet/browsers/portable_firefox * http://www.mozilla.com/firefox/all-older.html * http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox * http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows * http://kb.mozillazine.org/Shortcut_to_a_specific_profile