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Even though you can't uninstall it, you can disable the extension on a per-user basis. Firefox looks in both per-user and system-wide locations to find installed addons. Only per-user addons can be removed by the user, in the addons dialog. System-wide extensions can be disabled per-user, but not uninstalled (disabling is almost the same thing). To uninstall a system-wide extension, hopfully the publisher has an unistall script, but worst-case, you can just delete the extension folder from the system-wide location. e.g. C:\Users\All Users\Mozilla\something on Windows.