add firefox extension template

externs/webextension_polyfill/activitylog/OnExtensionActivityEvent.hx

@@ -0,0 +1,17 @@
+package webextension_polyfill.activitylog;
+
+/**
+	Receives an activityItem for each logging event.
+**/
+typedef OnExtensionActivityEvent = {
+	/**
+		Registers an event listener <em>callback</em> to an event.
+	**/
+	function addListener(callback:(details:OnExtensionActivityDetailsType) -> Void, id:String):Void;
+	/**
+		Deregisters an event listener <em>callback</em> from an event.
+	**/
+	function removeListener(callback:(details:OnExtensionActivityDetailsType) -> Void):Void;
+	function hasListener(callback:(details:OnExtensionActivityDetailsType) -> Void):Bool;
+	function hasListeners():Bool;
+};