Doesn't work on Flash game sites that have input bar focused —so it's useless like the userChrome.css trick. Make the full screen trigger without window focus. Nevermind, it's a glitch in Habbo site, I'am back to using this userChrome script: https://www.userchrome.org/what-is-userchrome-css.html

Superb. Works a treat and it does it quickly and looks absolutely perfect. Pressing ESC gets me back to the normal browser view, which is intuitive. There are many times, particularly, when I am writing or designing, that I like to go into total full screen mode (i.e. without the tabs and all the other clutter) and this does exactly that and does it more elegantly than anything else I have come across. Thank you for clearing up my screen so well :-)

UPDATE 2021-08-22:
Les Merlotta's trick for Mac from 2020-10 is still working on Firefox 91.0.1 Mac:
"Type "about:config" in your Firefox address bar (like going to a website), click "Accept the risk and continue". Then copy/ paste "full-screen-api.allow-trusted-requests-only" and change it to false."

OLDER COMMENT:
Thank you Trishul, good job. Running fine in macOS 10.10.13 High Sierra with Ctrl+Shift+F. I like that Cmd+Shift+F remains for the default full-screen behavior.

Just one request: please add an option page where to toggle if FS mode shall be left or kept when clicking onto a link (and maybe another option when leaving a domain).

It didn't work at first. But it works fine after running the Java script. Thanks for a very useful add-on.

As of 15 January 2018, this is the only add-on that will allow fullscreen functionality with no menu/nav bars on OSX 10.12.6 (FF 57.0.4). The browser.fullscreen.autohide function is broken for this machine configuration.

Feature request for this add-on: please add an icon that can be added to the menu bar that does the CTRL+SHIFT+F shortcut with one click.

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.

I'm afraid I've had the same problem, ctrl+shift+F doesn't work. Using Quantum - 57.0

*Update*: Can't work out how to reply to my own comment. Yes, I needed it for a presentation which I was doing in reveal.js

Didn't work for me. Full screen still had tabs and bar on FF 57 and macOS 10.13.1. Same experience on FF 68 and macOS 10.14.5. Cmd+Shift+F and Cmd+Ctrl+F switches to full screen. Ctrl+Shift+F has no effect.