add firefox extension template

externs/webextension_polyfill/webnavigation/OnCommittedEvent.hx

@@ -0,0 +1,19 @@
+package webextension_polyfill.webnavigation;
+
+/**
+	Fired when a navigation is committed. The document (and the resources it refers to, such as images and subframes)
+	might still be downloading, but at least part of the document has been received from the server and the browser has
+	decided to switch to the new document.
+**/
+typedef OnCommittedEvent = {
+	/**
+		Registers an event listener <em>callback</em> to an event.
+	**/
+	function addListener(callback:(details:OnCommittedDetailsType) -> Void, ?filters:EventUrlFilters):Void;
+	/**
+		Deregisters an event listener <em>callback</em> from an event.
+	**/
+	function removeListener(callback:(details:OnCommittedDetailsType) -> Void):Void;
+	function hasListener(callback:(details:OnCommittedDetailsType) -> Void):Bool;
+	function hasListeners():Bool;
+};