Bug 1734997 - Fix the setPriority-delayed-task test in Firefox r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D133497

testing/web-platform/tests/scheduler/task-controller-setPriority-delayed-task.any.js

@@ -16,7 +16,14 @@ promise_test(t => {
     assert_equals(++taskCount, 2);
 
     const elapsed = performance.now() - start;
-    assert_greater_than_equal(elapsed, 20);
+
+    if(navigator.userAgent.toLowerCase().indexOf('firefox') > -1){
+      // Firefox returns the timings with different precision,
+      // so we put 19 here.
+      assert_greater_than_equal(elapsed, 19);
+    } else {
+      assert_greater_than_equal(elapsed, 20);
+    }
   }, {signal: controller.signal, delay: 20});
 
   return Promise.all([task1, task2]);