I've got something that works, based on the classic userChrome.css technique that people used to use all the time on older versions of Firefox, and still works today if you enable it in about:config. It's based on a Gist link found on Reddit here and adapted to make it work on maximized windows.
Follow this guide on how to set up your userChrome...
- Adding the chrome directory in the right place in your profile folder
- Making the file
about:config so Firefox will use it when loaded.
Put the following code into your userChrome.css: