add firefox extension template

externs/webextension_polyfill/urlbar/EngagementState.hx

@@ -0,0 +1,9 @@
+package webextension_polyfill.urlbar;
+
+/**
+	The state of an engagement made with the urlbar by the user. <code>start</code>: The user has started an engagement.
+	<code>engagement</code>: The user has completed an engagement by picking a result. <code>abandonment</code>
+	: The user has abandoned their engagement, for example by blurring the urlbar. <code>discard</code>
+	: The engagement ended in a way that should be ignored by listeners.
+**/
+typedef EngagementState = String;