As I have discovered once you load your Firefox 2 profile into Firefox 3, it is not going to work correctly again in Firefox 2.  You will need to start a new profile if you want to go back to Firefox 2. You can however bring back over your bookmarks to Firefox 2.

Backup Bookmarks in Firefox 3

1. In Firefox 3 go to the Bookmarks Menu and select Organize Bookmarks
2. This will open the Library window.  From here click on Import and Backup then select Export HTML..
   
3. You will be prompted to select a location to save the bookmark list. Select the location then click ‘Save’

Importing Bookmarks into Firefox 2

1. In Firefox 2 go to the Bookmarks Menu and select Organize Bookmarks
2. This will open the Bookmarks Manager window. From go to the File menu and select Import…
   
3. Select ‘From file’ then click ‘Next’
4. You will be promoted to select the bookmark file you wish to import. Locate the Bookmarks.html file you saved earlier and click ‘Load’
5. Once the file is loaded your Book Manager window should look similar to the one below
   
6. Here you may want to do a little cleaning up.

   • The entries under the Mozilla Firefox folder (red arrow) are part of the Places bookmarking system and will not work with Firefox 2. You can safely remove those two entries.
   • If you were using the Bookmarks Toolbar feature, your old bookmarks are in bottom the Bookmarks Toolbar Folder (blue arrow). You will need to move all the entries listed in there to the top Bookmarks Toolbar Folder (green arrow).  After you have done this you can safely remove the blue arrow folder.
   • You will also notice a duplicate listing for Get Bookmark Add-ons, either one of these or both can safely be removed.