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.