I made the 3.6 to 7/8 switch only a few months ago.

Here are some extensions I found critical to my sanity to keeping things somewhat familiar:

   Status-4-Evar   <<<<<<------------ a must
   Add-ons Manager Dialog Returns
   Back/forward dropmarker
   Restore View Source for Firefox 6+

and

   Firefox 3 theme for Firefox 4+ (v2.0) 

Then once you are in FF8 treat yourself to extensions you can only running in FF8+ like Font Information which shows you what fonts are being used on the page in the page info dialog.

Now my FF8 looks a lot like 3.6 but works several times faster.