I don't know what happened but my list of extensions does not show up on the extensions tab of the Add-ons Manager on my Firefox 12.
Is there any
about:config settings to show the extensions or some fix?
I have closed Firefox, then I have deleted some files from profile folder (make a backup first):
extension.ini and extension.sqlite
Restart Firefox, it will ask to reinstall every extension.
Profile folder: "%APPDATA%\Mozilla\Firefox\Profiles\profile folder"
It worked for me.