If an extension worked in Firefox 4.0, it should be working in Firefox 5.0 as well -- Firefox 5 Compatibility Bump

You can force 5.0 "compatibility" with the "Compatibility Reporter" extension, or with extensions.checkCompatibility using about:addons.

When you reinstall a different version, at the end of the install, don't let the install start Firefox for you, rather exit, and then start Firefox in the normal manner to avoid inadvertently creating and using a new profile without your bookmarks, etc.

If you think your only alternative is going back to 4.0 or 4.0.1 you can get it at Firefox releases (all releases, including 4.0.1 and 5.0).

Do also check for Extension updates as many have already been bumped or updated by the authors recently on addons.mozilla.org

The link to all the FF releases is definitely NOT user friendly! What is the deal! Why is FF making it so difficult to use our addons! If it continues to be so difficult to use my toolbars such as Yahoo! and Norton toolbars, I will have to go back to IE or find a new browser... It's bad enough that there is an upgrade every couple of weeks!!

Yahoo and Norton have had months to get their acts together.

You should check your extensions individually. There may be an update on the developer's website. Often there are alternative extensions, sometimes better, certainly better maintained.