I've been in the habit of installing extensions and locking them via the group policy editor. This works great as the addons auto download, and the remove button is removed from the add-ons setting page in Firefox after the policy is applied. However, I've noticed that if you goto Firefox's addons website and search for an extension that is installed, that site gives you a remove button. If you click the button, the extension does get removed. Neither rebooting nor reapplying the group policy restores the extension even though the extension is still listed in the policy. They only way to reinstall it, is to manually browse for it on the site again and install it. It seems that removing the extension should not be possible given the policy.