Credit Tony E

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. See https://support.mozilla.com/kb/Uninstalling+Firefox

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

How do I uninstall Firefox 4?

I do not like that the Home button is at the far right now instead of the left. The human eyes naturally starts at the left. Why did you change that Mozilla? At least give users the option to move it where they like it. That kind of personalization functionality in the future would be very kewl.