Update (Sept. 29 3:45pm Pacific): A new update for Firefox that fixes this issue is now available, and all users are encouraged to update to the latest version from within the Firefox About window or by downloading from mozilla.org/firefox.
Update (Sept. 30 2:30pm Pacific): A new update for Thunderbird is also now available that fixes this issue for Thunderbird users.
We’ve identified an issue in which some users may have one or more of their add-ons hidden after upgrading to the latest Firefox version, affecting both desktop and mobile. These add-ons and their data are still intact and haven’t actually been removed. We paused new updates to Firefox to minimize the potential impact on users and will soon release an update to fix this issue and ensure all your add-ons are visible and usable. In the meantime, if you notice an add-on missing entirely from the Add-ons Manager, you can use a simple workaround to recover them.
We are working hard to resolve this issue. We know how important add-ons are to you and apologize for the inconvenience.
Justin Scott, on behalf of the Firefox Add-ons team