I just had to do this. I upgraded to Firefox 4 and videos wouldn’t play properly on some sites. I spent a little bit of time trying to resolve it but then decided it was faster to just go back to the earlier version where everything worked.
I exported all of my bookmarks to an HTML file first, uninstalled the new version, then re-installed the earlier version. I then went through and changed all of my settings, imported bookmarks, and re-installed my add-ons.
A bit of a pain, but it worked.
ETA: I only had problems with one computer. I have other computers running firefox 4 without any issues. YMMV.
In my experience, you don’t need to do this: Firefox 3.6 will use profiles created (or modified) by Firefox 4 just fine (and, of course, vice-versa). All you need to do is remove the old executable and start using the new one.
Rolling back from 4.0.1 will most likley work, but it’s possible that some upgraded extensions will only work on the newer browser, and you’ll have to manually go back and install the older version. Other than that, the team pretty much designed with falling back in mind, so it should work.
:smack: Gah! I have had MozBackup for a long time, and completely forgot all about it.
I tried FF4 and really saw no advantage, and some things that were new I did not care for, so I did go back to 3.6.7. Everything did work OK, but had to redo some sites that required ID and passwords. Each time I let FF “remember” them, but all this could have been avoided had I only remembered to do a MozBackup before hand.
BTW, this handy program backs up Thunderbird also.