Great! Rated 5 out of 5 stars
What I find most interesting about this extension is its "customizability". My own preferences and additional customizations, as well as experience with your version for Thunderbird would suggest the following options:
1. Display title in title bar the traditional way while still using the Firefox orange button (I do that with userChrome, so I have small tabs and still see the full title.
2 Display full menus under the Firefox button, just like you did under the Thunderbird button.
3. Luxury would be to have a few colour options just like the Thunderbird button has.