The solution for the original Disable F11 (https://support.mozilla.org/en-US/questions/1255696) is no longer working. When the solution is executed from the Browser Console... doc = Services.wm.getMostRecentWindow("navigator:browser").window.document; if(E=doc.getElementById('key_fullScreen')){E.removeAttribute('keycode');E.removeAttribute('command');} The return is a reference to "HTLMDocument chrome://browser/content/browser.xhtml" which can be opened up and displays more information. It used to just come back with something like "Undefined." and then it worked (F11 was disabled for the browser instance). Looks like it just started to not work with this latest version of 115.0 64-bit as this solution has been working for over a couple of years now. Is there a way to enable this to work in the latest version?

doc = Services.wm.getMostRecentWindow("navigator:browser").window.document;
if(E=doc.getElementById('key_enterFullScreen')){E.removeAttribute('keycode');E.removeAttribute('command');}
if(E=doc.getElementById('key_exitFullScreen')){E.removeAttribute('keycode');E.removeAttribute('command');}