In the patch for bug 918258, I was working on a mochitest, named test_bug448987.html. I had to use a setInterval timer there, namely: timer = setInterval(function() { if (counter > 10) { parent.ok(false, "Too often tried to focus image map, giving up"); parent.Simpletest.finish(); return; } synthesizeKey("VK_TAB", { shiftKey: shiftKeyOn }, window); shiftKeyOn = !shiftKeyOn; counter++; }, 100); Because for some reason, sending the tab key doesn't result in a focus move, just after the test is running (hence the retries with a limit of 10 times). This bug is about figuring out why this is occuring and how it can be fixed.