--- a/testing/marionette/client/marionette_driver/marionette.py
+++ b/testing/marionette/client/marionette_driver/marionette.py
@@ -874,25 +874,23 @@ class Marionette(object):
                                                                                   null, null),
                                                                                   attrs);
                 Services.perms.addFromPrincipal(principal, perm.type, perm.action);
                 return true;
                 """, script_args=(perm,))
 
         with self.using_context("content"):
             self.execute_async_script("""
-                let start = new Date();
-                let end = new Date(start.valueOf() + 5000);
                 let wait = function() {
-                  let now = new Date();
-                  if (window.wrappedJSObject.permChanged || end >= now) {
+                  if (window.wrappedJSObject.permChanged) {
                     marionetteScriptFinished();
+                  } else {
+                    window.setTimeout(wait, 100);
                   }
-                };
-                window.setTimeout(wait, 100);
+                }();
                 """, sandbox="system")
 
     @contextmanager
     def using_permissions(self, perms):
         '''
         Sets permissions for code being executed in a `with` block,
         and restores them on exit.