After updating to the latest release many users are reporting missing Firefox 7 Add-Ons, this behavior happens due to a bug found inside the latest Firefox release. The add-ons were not uninstalled, neither broken, don’t worry they still there and they are working, you just cannot see them. The Mozilla team already know about this issue and they are building a new update. They also have paused Firefox updates until they fix the problem to prevent from affecting even a bigger number of users. Just give them some time until this problem is fixed.
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.