This is aimed at anyone returning to Firefox and who don't like the website is now full screen warning when viewing HTML5 video.

• Navigate to about:config in Firefox (accept the warning that it gives you)

• Search for full-screen-api.warning.timeout and set the value from 3000 to 0

• The warning will not show up any more

Bonus tip: There is a transitional animation when you enter and exit full screen. If you don't like it:

• Set full-screen-api.transition-duration.enter to 0 0

• Set full-screen-api.transition-duration.leave to 0 0