It works (mostly) as intended. Four things to note:
1. control+shift+F is the keyboard shortcut, even on a Mac. This is actually very helpful. Since it does not override the default fullscreen behavior, both fullscreen modes are within reach (command+shift+F calls for the default fullscreen behavior).
2. Due to some restrictions it does not work on addons.mozilla.org. This might lead to some false reports saying it does not work.
3. As soon as you do anything in a webpage (click a link, search with Google, etc.), it will revert back to the default fullscreen behavior.
4. It requires access to "all data in all websites", so regular code audits might be needed to ensure that it does not do anything malicious.