If you want to learn more check the Mozilla Blog post Issue discovered with Firefox add-on upgrade.

Sept. 29, 2011 Update

In the meantime you can use a new Firefox add-on called Add-On Recovery Tool that fixes the problem by recovering the hidden add-ons.

Sept. 30, 2011 Update

Just a follow-up, Mozilla has updated Firefox 7 to 7.0.1. This latest version fixes the issue where some add-ons went gone after the update. You can read more about Mozilla Firefox 7 here, and you should also update to the latest version, here is the download link. All content on this site is provided with no warranties, express or implied. Use any information at your own risk. Always backup of your device and files before making any changes. Privacy policy info.