What exactly isn't working? 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. [https://addons.mozilla.org/en-US/firefox/addon/add-on-compatibility-reporter/ Add-on Compatibility Reporter :: Add-ons for Firefox] Please report back on your results.