In theory it is possible. I haven't tried this myself since 3.5 came out, so I don't know whether it works the same with 4. You need to install Firefox 3.6 and Firefox 4 to different program folders, and you need to create another profile under a different name (two running instances of Firefox cannot share a single profile). Finally, you want to edit your shortcuts to point to different profiles, and one of them should have the -no-remote startup switch. More info: [http://kb.mozillazine.org/Opening_a_new_instance_of_your_Mozilla_application_with_another_profile Opening a new instance of your Mozilla application with another profile - MozillaZine Knowledge Base] [http://support.mozilla.com/en-US/questions/795726 is it possible to run both firefox 3 and 4 in the same computer (windows) | Firefox Support Forum | Firefox Help] [http://forums.mozillazine.org/viewtopic.php?f=38&t=2179397 How run Version 4 and Version 3 simultaneously? • mozillaZine Forums]

Yes, this setup works with Firefox 4 for me.

Thank you very much. I appreciate the very quick response and the additional resource links. Excellent help!