Symantec need to update their Firefox add-ons so that they are compatible with Firefox 4. They have indicated that for Norton 360 they plan to release an update to Norton 360 to support Firefox 4 in early May - http://us.norton.com/support/kb/web_view.jsp?wv_type=public_web&docurl=20100720113635EN&ln=en_US

I do not know about the time scale for updates for other Norton products.

Pending the update by Symantec, 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.

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

Thanks Tony for your VERY prompt help!

You're welcome

I too was having problems with the 4.0 and wanted to go back to the 3.6 version. I don't claim to be a pro tech, however, instead of following the somewhat confusing instructions for uninstalling 4.0 as posted, I simply clicked the link to the 3.6 version and installed it. The 4.0 no longer exsist. All of my password, bookmarks, etc were transferred to the 3.6 from the 4.0. The 4.0 is no longer on my system. I am using Windows7. I didn't have to do all that other stuff in which I have no clue how to do or what he was talking about. I hope this helps all the others who like me; are not that techno savvy, and need help with this issue. Sometimes...simple is the best route to take.

By not removing the files I mentioned, you may have problems either now or later on if you decide to install Firefox 4 again.

The below link is what I had to do to completely remove Firefox and re-install a fresh "older" version of Firefox.

https://support.mozilla.com/en-US/questions/810726#answer-167109

After completely re-installing Firefox 3.6.8, I've had no more problems.

This is drastic , but it worked. Hope it helps.