When I first installed Firefox 4 it was REALLY buggy for me. Lots of crashes and lock ups. I then started removing plugins and its starting to behave much better now. I still have the occasional hiccup, but nothing like in the beginning. However, if you are STILL having crashes and lockups on FireFox, here are the steps to revert back to 3.6
Now before you panic and uninstall FireFox, try uninstalling a few plugins.
This is directly from the help people at mozilla help..
To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to “Remove my Firefox personal data”. If you select that option it will delete your bookmarks, passwords and other user data.
You can then install the latest version of Firefox 3.6 available from http://www.mozilla.com/en-US/firefox/all-older.html – it will automatically use your current bookmarks, passwords etc.
To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist – extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.
For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles