You can view disabled add-ons here:

orange Firefox button > Add-ons (check plugins and extensions)

or

Tools menu > Add-ons (check plugins and extensions)

If you don't like what you see, here's the process to roll back:

First, I recommend backing up your Firefox settings in case something goes wrong. See Backing up your information. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)

Next, download and save Firefox 3.6 to your desktop for future installation. http://www.mozilla.com/firefox/all-older

Close Firefox 4.

You could install Firefox 3.6 over it (many have reported success) or you could uninstall Firefox first. If you uninstall, do not remove your personal data and settings, just the program.

Unless you have installed an incompatible add-on, Firefox 3.6 should pick up where you left off. If there are serious issues, please post back with details.

Note: I haven't actually tried this myself!

You would think you would get some notice before it nukes 'ANYTHING 'on you computer. I've used Firefox for years and can't believe this. I would expect Microsoft to do something like this (like rebooting my computer in the middle of the night when my backups are running!), but I never thought Firefox would!

@jameslecomte, Add-ons contain a range of compatible Firefox versions, and Firefox honors the listed range by disabling the add-on if you are trying to run it on an unlisted version. I'm sure Norton will issue an update at some point.

I did as suggested above...used the link, http://www.mozilla.com/firefox/all-older downloaded 3.6 and just installed it, back to old ff with Norton toolbar. Thanks