Firefox 42 and previous versions offer a preference in about:config called full-screen-api.approval-required, by toggling its value to false we can able to disable YouTube.com is now fullscreen message. Some developments have happened in version 43:  Mozilla has improved full screen notifications experience in Firefox 43.

The dialog appears for a few seconds (unlike to earlier versions, doesn’t ask for permissions) and goes away automatically after that without needing any interaction from the user. But when the user moves the mouse or presses any key on the keyboard, full screen dialog appears again.

Firefox 43 Youtube.com is now full screen message

Mozilla removes the Old Search UI from Firefox

Firefox 43+: Remove ‘youtube.com is now full screen’ notification

Firefox 43+ doesn’t offer ‘full screen api approval required’ hidden preference, it has been removed. But you can still disable the message in version 43, here is how.

  1. Visit about:config
  2. Find the preference full-screen-api.warning.timeout and change its value to 0 from the default 3000.
  3. Done.

How to Restore Firefox’s Old Search UI and Preferences Window

Note: Remember this feature was designed in browsers to avoid phishing attacks, that’s a security measure, if it’s not bothering you, leave it.