To downgrade to Firefox 3.6 first uninstall Firefox 4, but do not select the option to "Remove my Firefox personal data". If you select that option it will delete your bookmarks, passwords and other user data. For details on uninstalling see https://support.mozilla.com/kb/uninstalling+Firefox

You can then install the latest version of Firefox 3.6 available from http://www.mozilla.com/en-US/firefox/all-older.html - it will automatically use your current bookmarks, passwords etc.

To avoid possible problems with downgrading, I recommend going to your profile folder and deleting the following files if they exist - extensions.cache, extensions.rdf, extensions.ini, extensions.sqlite and localstore.rdf. Deleting these files will force Firefox to rebuild the list of installed extensions, checking their compatibility, and reset toolbar customizations.

For details of how to find your profile folder see https://support.mozilla.com/kb/Profiles

Thank you sooooo much! I've had so many problems with running Firefox 4 on Windows 7. I am very glad to get back to an earlier version that actually works!

Thank you, Tony E for helping me with this. I have not got the mental energy at this time to work out how to make Firefox 4 work so that I can listen to a radio station online, and have successfully worked out how to go back to Firefox 3.6 and can now listen successfully. Your instructions were clear for a limited computer user!!!! Thank you once again.