Capture the shift key when pressed

I am writing a program that requires me to detect when a shift key is pressed and do something. Something like window.addEventListener('keydown', event => console.log(event), false); // expect to see an event logged to console when pushing the "shift" key This works great on all browsers except Firefox. "alt" and "fn" keys are also not able to be detected on Firefox. Is there some sort of advanced security settings I can disable to let me capture these things? Thanks in advance.