I'll be glad to try to try to help you figure out why Firefox 4 is slower for you. It should be faster and possibly something is going wrong that we can fix.

One thing to try is to try to start it in safe mode. You can see the article here for how to do that: http://support.mozilla.com/en-US/kb/Safe%20Mode

But if you really want to go back to Firefox 3.6, you can simply download it from here and install it: http://www.mozilla.com/en-US/firefox/all-older

I'm going back to 3.6! 4.0 has junked a vital feature--the ability to 'drag and drop' URLs from the address bar to the bookmarks toolbar. Instead, one has to click the star and play with menus. Slow. Worse, that always dumps the URL at the bottom of the list. In 3.6, I could, with one 'held-mouse-click,' put the URL anywhere in the drop-down list below a toolbar bookmark folder. Also, 4.0 expanded the size of toolbar bookmarks, so I can't see as many of them. Bother. Dear Mozilla, did you hire a Microsoft employee, for 'upgrades to worse?'