add firefox extension template

externs/webextension_polyfill/runtime/BrowserInfo.hx

@@ -0,0 +1,23 @@
+package webextension_polyfill.runtime;
+
+/**
+	An object containing information about the current browser.
+**/
+typedef BrowserInfo = {
+	/**
+		The name of the browser, for example 'Firefox'.
+	**/
+	var name : String;
+	/**
+		The name of the browser vendor, for example 'Mozilla'.
+	**/
+	var vendor : String;
+	/**
+		The browser's version, for example '42.0.0' or '0.8.1pre'.
+	**/
+	var version : String;
+	/**
+		The browser's build ID/date, for example '20160101'.
+	**/
+	var buildID : String;
+};