How to remove all bars in fullscreen mode in Firefox on MacOS/OSX ("kiosk mode")

Procedure

If you want to remove all toolbars from Firefox in fullscreen mode on Mac, this is the procedure:

1. Type about:config: in the location bar to open advanced settings. (If it is the first time you do it, you might have to click "confirm" or something like that in the scary screen that comes up first.)
2. search for full-screen-api.allow-trusted-requests-only and double-click the line that is found, so the value changes to false
3. Bring up the Bookmark Toolbar if you don't have it (menu View -> Toolbars -> Bookmark Toolbar)
4. Right-click on empty space in the Bookmark Toolbar, and select New Bookmark...
5. In the Name field, type something like "Enter Fullscreen" (it doesn't really matter, just don't leave it blank)
6. In the Location field, enter the following line:

   javascript:document.getElementsByTagName('html')[0].mozRequestFullScreen();void(0)

7. Click Add to save the bookmark.

Now, when you click on that bookmark, Firefox will go "real fullscreen", hiding all toolbars. To get them back, you will have to press ESC.

Now, there is a bit of a snag. That preference in about:config: is set to true by default for a reason: it is a bit unsafe to allow any website to use fullscreen-enabling javascript. For this reason I would suggest to only do this for limited periods, then switch it back once you're done.

Long-winded explanation

FF has a preference, browser.fullscreen.autohide, that in theory should govern the behaviour of toolbars when going fullscreen. It is set to true by default, meaning toolbars should disappear. However, for some reason this property has (almost) always failed to work on MacOS/OSX builds of Firefox.

There used to be a few extensions that corrected that behaviour, but they've all gone away when Firefox switched to WebExtension APIs, which basically sandboxed extensions more rigidly and blocked them from changing the browser interface as deeply as they could before. This was the price for the dramatic performance improvement seen in Firefox 57+.

This workaround hence relies on the new Fullscreen WebAPI, which is supposed to be a new, standard way to enable fullscreen via Javascript on any browser (well, any browser that implements it - just a few at the moment). However, because of security concerns, this API is locked down by default; so, in order to use it, we first have to relax checks as described above. Once that is done, we can launch a bit of JS from a bookmark that triggers "real" fullscreen.

Credit to this AskDifferent post where I first found the trick.