When I upgraded to Firefox 7, all my add-ons (except for a few) were lost. Is there any way I can recover those add-ons (or at least find out what add-ons I had installed before the upgrade broke them)? I have tried upgrading to FF 7.01, and using the add-on recovery dialog, and neither of those have worked.
AFAIK, if the addons, and the folders they were in are gone, then there is no way to recover the addons you had. But i can suggest a way to prevent it from happening again.
In firefox, you can actually create "collections" of your favorite addons, so you can then install them elsewhere, or simply share them.
more info here:
Basically, you are going to add any addon you want, and save them to a personal page, then you can make the collection public if you want.
There was an addon itself that let you automatically make collections of your currently installed addons, and it updated itself any time you installed a new addon. But i cannot find it. It may be no longer available.
Of course, this method will not save your addons preferences, is just a way to "bookmark" your addons, so you can access them any time later (from any computer).
If you want to backup the addons AND their preferences, then they are other ways to do it, like FEBE mentioned in a comment before.
Firefox doesn't delete the non-compatible add-ons, it just disables them. You should see a list of all your add-ons, working and not-working in the 'add-on' tab. You can get to the Add-ons Manager by typing "about:addons" in the location bar. Firefox should also have checked to see if newer versions were available and installed them for you automatically too. If it didn't, either you told it not to, or there aren't any working (or compatible) versions available.
If there are no compatible versions available, you CAN make them work by editing the version limits in the 'install.rdf' file within the add-on/extension. This is not recommended, but CAN work, although by doing this, you automatically agree that we don't have to help you fix things if it goes bad. :) No, I won't explain how.