Fullscreen handler not setup properly for Firefox

[mirisuzanne]

Describe the bug

Firefox is refusing to put slides into fullscreen mode:

Request for fullscreen was denied because Element.requestFullscreen() was not called from inside a short running user-generated event handler.

To Reproduce

1. Open in Firefox
2. Use the shortcut to enter fullscreen mode (depends on branch)
3. Check console for error

Expected behaviour

Enter fullscreen mode.

Additional context

Fullscreen works in Chromium & Webkit browsers.

[mirisuzanne]

I thought 70a0411 might fix this, but it did not. Do we need a 'shorter lived' listener specifically for the fullscreen request? This is supposed to happen 'within 1s' - and it clearly is, so there must be some other issue?