Works flawlessly for me! (as of Feb 2019, FF 65.0)

I don't get what some of the other reviews are complaining about, because I'm really glad to finally have true fullscreen again in Firefox.

One limitation is indeed that you can't change the keyboard shortcut in the addon settings, but I solved it by using an external tool (Karabiner), mapped the standard cmd-shift-f to the addon's ctrl-shift-f shortcut.

Summary: Get this addon if you want real fullscreen back! And, thanks a lot Trishul for this addon.