Fix #13335. Remove "use strict".

src/.jshintrc

@@ -4,7 +4,6 @@
 	"newcap": false,
 	"quotmark": "double",
 	"regexdash": true,
-	"strict": true,
 	"trailing": true,
 	"undef": true,
 	"unused": true,

src/intro.js

@@ -12,4 +12,9 @@
  * Date: @DATE
  */
 (function( window, undefined ) {
-"use strict";
+
+// Can't do this because several apps including ASP.NET trace
+// the stack via arguments.caller.callee and Firefox dies if
+// you try to trace through "use strict" call chains. (#13335)
+// Support: Firefox 18+
+//"use strict";