Browser Console Script to Update Copy Link accelerator key to A (which also can be used for other context menu hotkeys)

menuHotkeyChanger.js

/* Script for the Browser Console
	
	   NOTE: BEFORE RUNNING THIS SCRIPT, CHECK THIS SETTING:
	   Type or paste about:config into the address bar and press Enter
	   Click the button promising to be careful
	   In the search box paste devtools.chrome.enabled
	   If the preference is false, double-click it to toggle to true
	
	Paste this entire script into the command line at the bottom of the Browser Console (Windows: Ctrl+Shift+j)
	Then press Enter to run the script.
*/

(function(){
	// Define an array of key change objects (so far, only one)
	var myKeyChanges = [
		{
			id: 'context-copylink', 
			newkey: 'a',
			newlabel: 'Copy Link Location'
		},
		{
			id: 'context-copyemail', 
			newkey: 'A',
			newlabel: 'Copy Email Address'
		}
	];
	// Current window
	for (var i=0; i<myKeyChanges.length; i++){
		var menuitem = gBrowser.ownerDocument.getElementById(myKeyChanges[i].id);
		if (menuitem){
			if (myKeyChanges[i].newkey.length == 1){
				menuitem.setAttribute('accesskey', myKeyChanges[i].newkey);
			}
			if (myKeyChanges[i].newlabel.length > 0){
				menuitem.setAttribute('label', myKeyChanges[i].newlabel);
			}
		}
	}
	// All future windows -- based largely on
	// https://www.reddit.com/r/firefox/comments/kilmm2/restore_ctrlshiftb_library_by_setting_configjs/
	var acceleratorUpdater = {
		observe: function (aSubject) {
			aSubject.addEventListener('DOMContentLoaded', this, {once: true});
		},
		handleEvent: function (aEvent) {
			let document = aEvent.originalTarget; 
			let window = document.defaultView; 
			let location = window.location;
			if (/^(chrome:(?!\/\/(global\/content\/commonDialog|browser\/content\/webext-panels)\.x?html)|about:(?!blank))/i.test(location.href)) {
				if (window._gBrowser) {
					for (var i=0; i<myKeyChanges.length; i++){
						var menuitem = window.document.getElementById(myKeyChanges[i].id);
						if (menuitem){
							if (myKeyChanges[i].newkey.length == 1){
								menuitem.setAttribute('accesskey', myKeyChanges[i].newkey);
							}
							if (myKeyChanges[i].newlabel.length > 0){
								menuitem.setAttribute('label', myKeyChanges[i].newlabel);
							}
						}
					}
				}
			}
		}
	}
	Services.obs.addObserver(acceleratorUpdater, "chrome-document-global-created", false);
})();

Make sure you are using the Browser Console and not the in-tab Web Console

I've been using Firefox since I started learning how to code (back in college?) but I just learned today that the Browser Console (CTRL+SHIFT+J) is not the same as in-tab Web Console (CTRL+SHIFT+I).

The [program folder] referenced in the script is the directory where Firefox's program files are installed -- not profile specific.

Thanks for clarifying. So what I found out, in Linux this path is located in /usr/lib/firefox (added files in /usr/lib/firefox/defaults/pref)

EDIT:
was getting parse error and i realized the config.js was on the wrong folder. Fixed by following steps above:

• config-prefs.js is located in /usr/lib/firefox/defaults/pref/
• config.js is in /usr/lib/firefox/

Followed your steps and it's working correctly now for me

Cheers