Due to security vulnerabilities, rolling back to Firefox 4.0.1 is not recommended.

If Firefox disabled some add-ons (especially extensions), it likely is because they include a list of compatible Firefox versions and haven't yet been updated for Firefox 5. If they worked on Firefox 4, there is a very good chance they will work on Firefox 5.

You can force Firefox to ignore add-on version restrictions by (what else) installing an add-on. Then you can test whether the add-ons important to you actually work in Firefox 5.

Add-on Compatibility Reporter :: Add-ons for Firefox

Please report back on your results.

Firefox forcing 90% of users to disable compatibility verification probably introduces just as many security vulnerabilities, but I guess that's the only choice we've got if we still want to use the Google Toolbar and other extensions, since there was virtually no time for developers to verify and re-release their add-ons between 4.0 and 5.0.

I am using and abusing the Add-on Compatibility Reporter extension too. Just don't forget to restart Firefox twice after installing it, once for the extension itself, the second time to enable the so-called incompatible extensions it detected.

Install Firefox 5.0 first and then install the recent Norton update patch. Some say installing the Firefox 5.0 after the Norton update may not have the toolbar working properly in Firefox 5.0

Unlike in past with taking forever to support 4.0 and then 4.0, Norton has been rather quick in updating for the Firefox 5.0 released on June 21 (not earlier like some jump the gun sites claim).

http://community.norton.com/t5/Norton-Internet-Security-Norton/Update-Available-Firefox-5-0-Compatibility-Release-for-Norton/td-p/478160

Despite incorrect claims in first pages of thread, the Firefox 5.0 was not actually released until June 21 as some sites were jumping the gun as early as June 17. http://community.norton.com/t5/Norton-Internet-Security-Norton/Norton-Toolbar-for-Firefox-5/td-p/475704