If you still don’t know the news: Firefox with new UI, Australis is available on Nightly build and we’ve covered Classic Theme Restorer add-on that restores its interface to classic one which also includes Menu button. But do you know :Australis it self offers menu Bar that existed on Firefox older versions, if you don’t like new menu you can enable that menu bar and use that instead of new one.
Enable Menu bar and bookmarks bar in Firefox Australis
To enable menu bar and bookmarks bar, right click on place where Firefox menu button used to exist and select "Menu bar" and "bookmarks bar". Hope this menu is much useful than new menu offered on right extreme of browser.