You or someone else might need to port the add-on from using a URL bar button to a regular button. It might be as simple as changing page_action to browser_action in the manifest.json file (https://github.com/jscher2000/cache-favicons-for-bookmarks-extension/blob/master/manifest.json) but I haven't tested it. (**EDIT** the "show_matches" key isn't supported for browser_action, so that would need to be removed.)