_template-ff-addon-BootstrapWatchHostEventListener - A template for bootstrapped Firefox addon that uses event listeners to watch for page loads and when a page load matches host that you specify it will inject to it.

bootstrap.js

const {interfaces: Ci,	utils: Cu} = Components;
Cu.import('resource://gre/modules/Services.jsm');
const ignoreFrames = true;

function addDiv(theDoc) {
	Cu.reportError('addDiv host = ' + theDoc.location.host);
	if (!theDoc) { Cu.reportError('no doc!'); return; } //document not provided, it is undefined likely
	if(!(theDoc.location && theDoc.location.host.indexOf('github.com') > -1)) { Cu.reportError('location not match host:' + theDoc.location.host); return; }
	
	//if (!theDoc instanceof Ci.nsIDOMHTMLDocument) { Cu.reportError('not html doc'); return; } //not html document, so its likely an xul document //you probably dont need this check, checking host is enought
	Cu.reportError('host pass');

	//add your stuff here
	
}

function removeDiv(theDoc, skipChecks) {
	//Cu.reportError('removeDiv');
	if (!skipChecks) {
		if (!theDoc) { Cu.reportError('no doc!'); return; } //document not provided, it is undefined likely
		if(!(theDoc.location && theDoc.location.host.indexOf('github.com') > -1)) { Cu.reportError('location not match host:' + theDoc.location.host); return; }
		
				//if (!theDoc instanceof Ci.nsIDOMHTMLDocument) { Cu.reportError('not html doc'); return; } //not html document, so its likely an xul document //you probably dont need this check, checking host is enought
	}
	
	var alreadyThere = false; //change this to some test to see if the stuff needing adding is already there
	if (alreadyThere) {
		Cu.reportError('stuff found in document');
		//run to remove the stuff
	}
	//else its not there so no need to do anything
}

function listenPageLoad(event) {
	var win = event.originalTarget.defaultView;
	var doc = win.document;
	Cu.reportError('page loaded loc = ' + doc.location);
	if (win.frameElement) {
		//its a frame
		Cu.reportError('its a frame');
		if (ignoreFrames) {
			return;//dont want to watch frames
		}
	}
	addDiv(doc);
}

/*start - windowlistener*/
var windowListener = {
	//DO NOT EDIT HERE
	onOpenWindow: function (aXULWindow) {
		// Wait for the window to finish loading
		let aDOMWindow = aXULWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
		aDOMWindow.addEventListener("load", function () {
			aDOMWindow.removeEventListener("load", arguments.callee, false);
			windowListener.loadIntoWindow(aDOMWindow, aXULWindow);
		}, false);
	},
	onCloseWindow: function (aXULWindow) {},
	onWindowTitleChange: function (aXULWindow, aNewTitle) {},
	register: function () {
		// Load into any existing windows
		let XULWindows = Services.wm.getXULWindowEnumerator(null);
		while (XULWindows.hasMoreElements()) {
			let aXULWindow = XULWindows.getNext();
			let aDOMWindow = aXULWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
			windowListener.loadIntoWindow(aDOMWindow, aXULWindow);
		}
		// Listen to new windows
		Services.wm.addListener(windowListener);
	},
	unregister: function () {
		// Unload from any existing windows
		let XULWindows = Services.wm.getXULWindowEnumerator(null);
		while (XULWindows.hasMoreElements()) {
			let aXULWindow = XULWindows.getNext();
			let aDOMWindow = aXULWindow.QueryInterface(Ci.nsIInterfaceRequestor).getInterface(Ci.nsIDOMWindowInternal || Ci.nsIDOMWindow);
			windowListener.unloadFromWindow(aDOMWindow, aXULWindow);
		}
		//Stop listening so future added windows dont get this attached
		Services.wm.removeListener(windowListener);
	},
	//END - DO NOT EDIT HERE
	loadIntoWindow: function (aDOMWindow, aXULWindow) {
		if (!aDOMWindow) {
			return;
		}
		if (aDOMWindow.gBrowser) {
			aDOMWindow.gBrowser.addEventListener('DOMContentLoaded', listenPageLoad, false);
			if (aDOMWindow.gBrowser.tabContainer) {
				//has tabContainer
				//start - go through all tabs in this window we just added to
				var tabs = aDOMWindow.gBrowser.tabContainer.childNodes;
				for (var i = 0; i < tabs.length; i++) {
					Cu.reportError('DOING tab: ' + i);
					var tabBrowser = tabs[i].linkedBrowser;
					var win = tabBrowser.contentWindow;
					loadIntoContentWindowAndItsFrames(win);
				}
				//end - go through all tabs in this window we just added to
			} else {
				//does not have tabContainer
				var win = aDOMWindow.gBrowser.contentWindow;
				loadIntoContentWindowAndItsFrames(win);
			}
		} else {
			//window does not have gBrowser
		}
	},
	unloadFromWindow: function (aDOMWindow, aXULWindow) {
		if (!aDOMWindow) {
			return;
		}
		if (aDOMWindow.gBrowser) {
			aDOMWindow.gBrowser.removeEventListener('DOMContentLoaded', listenPageLoad, false);
			if (aDOMWindow.gBrowser.tabContainer) {
				//has tabContainer
				//start - go through all tabs in this window we just added to
				var tabs = aDOMWindow.gBrowser.tabContainer.childNodes;
				for (var i = 0; i < tabs.length; i++) {
					Cu.reportError('DOING tab: ' + i);
					var tabBrowser = tabs[i].linkedBrowser;
					var win = tabBrowser.contentWindow;
					unloadFromContentWindowAndItsFrames(win);
				}
				//end - go through all tabs in this window we just added to
			} else {
				//does not have tabContainer
				var win = aDOMWindow.gBrowser.contentWindow;
				unloadFromContentWindowAndItsFrames(win);
			}
		} else {
			//window does not have gBrowser
		}
	}
};
/*end - windowlistener*/

function loadIntoContentWindowAndItsFrames(theWin) {
	var frames = theWin.frames;
	var winArr = [theWin];
	for (var j = 0; j < frames.length; j++) {
		winArr.push(frames[j].window);
	}
	Cu.reportError('# of frames in tab: ' + frames.length);
	for (var j = 0; j < winArr.length; j++) {
		if (j == 0) {
			Cu.reportError('**checking win: ' + j + ' location = ' + winArr[j].document.location);
		} else {
			Cu.reportError('**checking frame win: ' + j + ' location = ' + winArr[j].document.location);
		}
		var doc = winArr[j].document;
		//START - edit below here
		addDiv(doc);
		if (ignoreFrames) {
			break;
		}
		//END - edit above here
	}
}

function unloadFromContentWindowAndItsFrames(theWin) {
	var frames = theWin.frames;
	var winArr = [theWin];
	for (var j = 0; j < frames.length; j++) {
		winArr.push(frames[j].window);
	}
	Cu.reportError('# of frames in tab: ' + frames.length);
	for (var j = 0; j < winArr.length; j++) {
		if (j == 0) {
			Cu.reportError('**checking win: ' + j + ' location = ' + winArr[j].document.location);
		} else {
			Cu.reportError('**checking frame win: ' + j + ' location = ' + winArr[j].document.location);
		}
		var doc = winArr[j].document;
		//START - edit below here
		removeDiv(doc);
		if (ignoreFrames) {
			break;
		}
		//END - edit above here
	}
}

function startup(aData, aReason) {
	windowListener.register();
}

function shutdown(aData, aReason) {
	if (aReason == APP_SHUTDOWN) return;
	windowListener.unregister();
}

function install() {}

function uninstall() {}

delete