I mentioned that I had a project assigned to try and get multiple browsers running to the same user at the same time so that we could run the release supported by the various vendors on our intranet. One doesn't want to allow older browsers and versions of Java to get out to the Internet. Behold, three versions running in one session:
In this case, a user can use the Oracle and SAP software and currently surf the Internet using Firefox 3, 4 and 5.
I had to think about the best way to make this work, so the current version always works normally and makes use of the users bookmarks and settings (obviously). The older versions run as users "firefox3" and "firefox4" and I made use of the -ProfileManager feature in Firefox to allow multiple users to run from the same user account. I then locked down prefs.js and localstore.rdf to settings I want to be used. For instance in order to make it look more like "software" I hid the URL and menu bars so they aren't tempted to try and surf with the sessions. The beauty of this design is that when Firefox 3 and 4 are no longer needed, I simply delete the user account and all of of these settings and infrastructure for that version is deleted.
If you are struggling with the same issue, this technique allows you to run all of your Firefox sessions from the same server. Our server for Firefox is a monster, and it would have been a shame to have to offload this functionality to another server or VM instance.