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

What problems are you having with Firefox 4?

Well... I'm not Jactwist, but I think I may answer in his place. Firefox 4 has a lot of bugs. - It is impossible to print - https://support.mozilla.com/pl/questions/803058 - After adding a bookmark all the icons (back, forward, home, adblock) get deformed and stretched - It is much more slower than the previous version - When closed, it doesn't really shut down (in the task manager I can still see firefox.exe using a sick amount of memory - hundreds of megabytes) and it is impossible to open it again without forcing the shutdown of the process in the task manager

I cannot understand why Mozilla foundation released this version when it is obviously underdone...