_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() {}

README

Rev1

• Currently setup to search for github.com in the host name
• addDiv function is where you want to edit to add, be sure to edit removeDiv function to remove the stuff you added to the document
• The skipChecks parameter of removeDiv is meant only for if running removeDiv in addDiv function. For example you want to ensure that the old stuff is removed before adding the new stuff so you run removeDiv(theDoc,true) before adding

delete