Disable firefox fullscreen warning with html5 stuff?

This is the security feature, but yes it's annoying if you're on trusted site and the message popups every time you enter full screen.

Solution: enter about:config in address-bar, and search for full-screen-api.approval-required and set it to false

You're done!

UPDATE: OCT-2019 to cover Firefox Quantum:

Enter about:config in Firefox address-bar and search for the keyword "full-screen". Identify following listing and change their respective values as mentioned against them below:

full-screen-api.transition-duration.enter - 0 0
full-screen-api.transition-duration.leave - 0 0
full-screen-api.transition.timeout - 0
full-screen-api.warning.delay - 0
full-screen-api.warning.timeout - 0

(Archival Notes) FireFox 42

Option "full-screen-api.approval-required", no longer hides the Website is Now Full Screen warning. In previous versions setting it to "false" could resolve the problem. They are putting a new option in i.e. (full-screen-api.warning.timeout "0") that will do the same thing, but it will be rolled out in FireFox 43.

You can change the fade in/out animation when going full-screen by setting:

full-screen-api.transition-duration.enter 0 0

full-screen-api.transition-duration.leave 0 0

Note/Warning - Defaults are 200 200 and 0 0 will remove them completely.

Keep in mind these are put in because they are a security features to help stop Phishing Attacks. So either wait till Firefox should enable "full-screen-api.warning.timeout" option in next few releases OR use above at your own risk...