add firefox extension template

externs/webextension_polyfill/cookies/CookieStore.hx

@@ -0,0 +1,20 @@
+package webextension_polyfill.cookies;
+
+/**
+	Represents a cookie store in the browser. An incognito mode window, for instance, uses a separate cookie store from a
+	non-incognito window.
+**/
+typedef CookieStore = {
+	/**
+		The unique identifier for the cookie store.
+	**/
+	var id : String;
+	/**
+		Identifiers of all the browser tabs that share this cookie store.
+	**/
+	var tabIds : Array<Float>;
+	/**
+		Indicates if this is an incognito cookie store
+	**/
+	var incognito : Bool;
+};