How Can I Hide All Toolbars When Firefox is in Fullscreen?

You can advantage of HTML5 Full Screen API.

Paste code below into console and click big black box to activate fullscreen mode:

(function() {
  var el = document.createElement('div'),
      docEl = document.documentElement;

  el.innerText = 'Go to fullscreen view';
  el.setAttribute('style', 'position: fixed; top: 10%; left: 10%; padding: 30%; background: #000; color: #fff; opacity: .7; cursor: pointer;')
  document.body.appendChild(el)

  el.onclick = function() {
    if (docEl.requestFullscreen) {
      docEl.requestFullscreen();
    } else if (docEl.mozRequestFullScreen) {
      docEl.mozRequestFullScreen();
    } else if (docEl.webkitRequestFullscreen) {
      docEl.webkitRequestFullscreen();
    }
    document.body.removeChild(el);
  };
})();

...or use bookmarklet:

javascript:(function(){var e=document.createElement("div"),t=document.documentElement;e.innerText="Go to fullscreen view";e.setAttribute("style","position: fixed; top: 10%; left: 10%; padding: 30%; background: #000; color: #fff; opacity: .7; cursor: pointer;");document.body.appendChild(e);e.onclick=function(){if(t.requestFullscreen){t.requestFullscreen()}else if(t.mozRequestFullScreen){t.mozRequestFullScreen()}else if(t.webkitRequestFullscreen){t.webkitRequestFullscreen()}document.body.removeChild(e)}})();

As lexu mentioned, what you want is typically called "kiosk mode". To my knowledge, Firefox does not include built-in kiosk mode functionality (I think only IE includes it in the base version), but it is available with a couple of different Firefox plugins.

R-Kiosk is perhaps the most popular one. I've used it before in one of my previous jobs with a Firefox 3.x setup, but I haven't tried it in a couple of years so I don't know about compatibility and functionality with newer browsers. It does state it works through the latest versions of Firefox.

The other fairly popular one is FF Fullscreen which I haven't tried, but doesn't appear to be a full kiosk mode that prevents users from getting to the desktop, but rather just a fullscreen with no toolbars mode, which might be what you're looking for anyway. Your question didn't state whether or not you wanted to keep users from getting to the desktop.

An alternative is the 'Old Lion Fullscreen' addon. If works reliably (unlike FF Fullscreen which seems to have unpredictable full-screen behavior). The only drawback is that the menubar does not appear when hovering near the top.

Link : https://addons.mozilla.org/en-US/firefox/addon/old-lion-fullscreen/