add firefox extension template

externs/webextension_polyfill/topsites/MostVisitedURL.hx

@@ -0,0 +1,29 @@
+package webextension_polyfill.topsites;
+
+/**
+	An object encapsulating a most visited URL, such as the URLs on the new tab page.
+**/
+typedef MostVisitedURL = {
+	/**
+		The most visited URL.
+	**/
+	var url : String;
+	/**
+		The title of the page.
+		Optional.
+	**/
+	@:optional
+	var title : String;
+	/**
+		Data URL for the favicon, if available.
+		Optional.
+	**/
+	@:optional
+	var favicon : String;
+	/**
+		The entry type, either <code>url</code> for a normal page link, or <code>search</code> for a search shortcut.
+		Optional.
+	**/
+	@:optional
+	var type : MostVisitedURLTypeEnum;
+};