I deleted some apps in Firefox 4, opened 3.6, closed it, and opened 4 again. But now Firefox 4 is having some severe problems.

Question

After doing what I described in the thread title, Firefox 4 presented the following issues:
1- Tabs would not close.
2- The "forward" and "back" button would not be lit or clickable, as if I didn't visit any websites. Although pages did load, I could not go back and forth between them via these buttons.
3- the "feedback" button in the right-hand side of the screen was completely gone. Trying to download the respective feedback app that Mozilla provides didn't work, it wouldn't download.
4- the App Management screen would not open.
5- I did not save any of the pages from the previous session. So I closed FF 4 and opened 3.6 once again (wich appears to be functioning normally) and the pages came back
So basically my FF 4 got completely messed up. Reinstalling it did nothing. Cleaning the registry with Registry Mechanic was to no avail either. I had to uninstall FF4 since it was unusable.
Possibly useful fact: The apps I had removed in FF 4 were also removed in 3.6, wich indicates that maybe the FF 4 installation did not create copies of it's own for the bookmarks, apps, cache etc, so I am afraid to delete those when the FF 4 uninstallation wizard prompts me to, as it would also probably delete the stuff I have in 3.6 .
== This happened ==
Every time Firefox opened
== Started after using Firefox 3.6 then switching to 4 again.

Answer 1

I assume by apps you mean add-ons or extensions or plugins. These are held in the profile.
Unless you set up a new profile, then FF4 and FF3 would be using the same one.
http://kb.mozillazine.org/Profile_folder_-_Firefox
When I test a beta I always use a new profile, or update one from a previous version, but I would never expect the profile to be usable again in a previous version of Firefox.
Since FF4 has a new add-on manager it doesn't surprise me that FF 3 won't work with a profile that's been modified in FF 4.