It seems there is no way to see permissions of an installed add-on at the moment within Firefox? AMO lists the permissions used by an add-on, but there are multiple clicks to get to that page from the manager. Also, the version listed on AMO may not match the one installed locally, making it even harder to figure out the current state. There are also addons installed from outside AMO, where you would not able to know what permissions are used unless you uninstall and reinstall it. So I think it would probably be helpful if the manager can display the permissions directly in the details page.