Firefox test

Open this page in Firefox. Tap play on each video. Note the numbers that open the full screen.

1 · YouTube as today

The player as it is live today.

2 · YouTube with the full-screen permission

Today's player; the page allows it the full screen.

3 · YouTube with its own full-screen button

Tap play, then YouTube's full-screen button at the bottom right.

4 · Vimeo as today

For comparison: it opens the full screen in Firefox today.

5 · A plain video file

The browser's own video, no YouTube.