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?

Thx Tony! It's nice to receive a direct, informative and usable response! To answer your question: I need to evaluate to what extent my reaction to 4.0 is frustration driven, and intend to learn more about it before switching back. Seems as if there have been several elements of disappointment - one being the inability to scroll down a page from the keyboard controllers without first activating the cursor on the page (without doing so, the keyboard command shoots you straight to the bottom on the entry). Thx again. Have a great day! Incidentally, I'm impressed (and envious) with your technical insight!