Update: The preference mentioned below has been removed since FF42. In FF43 there is a new one available that will do the job.
Setting the value of ‘full-screen-api.warning.timeout‘ to zero (0) ms will disable the message again.
Blame it to the HTML5 fullscreen API, once you’ve entered fullscreen, you’re asked to confirm or deny the action.
… is now fullscreen. Press ESC at any time to exit. Deny / Allow *Remember decision for …
Even when you have approved and remembered this decision for a specific domain, firefox shows a annoying three seconds message.
… is now fullscreen. Press ESC at any time to exit.
Here’s how to permanently disable this approval request or message:
- enter ‘about:config’ into address line
- search for ‘full-screen-api.approval-required‘
- set attribute value ‘true’ to ‘false’
That’s it!
Note: Keep in mind, this alert message during full-screen mode is a security feature. Think twice if you are going to disable this behavior.