I cannot activate or uninstall some add ons such as Stumble On and Morning Coffee
After one of the upgrades of FireFox the Stumbleon and Morning Coffee addons stopped working. In the tools section it says that they are not compatible but in the Firefox addon site it says they are.
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You can delete the files extensions.* (extensions.rdf, extensions.cache, extensions.ini, extensions.sqlite) and compatibility.ini in the Firefox profile folder to reset the extensions registry. New files will be created when required.
See "Corrupt extension files": http://kb.mozillazine.org/Unable_to_install_themes_or_extensions
If you see disabled extensions that are not compatible on the next start in "Tools > Add-ons > Extensions" then click the "Find Updates" button to do a compatibility check.