Some malicious websites force Firefox into full-screen mode the moment their page loads, for example:
Less-technical users may miss the brief "Press ESC to exit full-screen mode" message, and then be stuck: not knowing either ESC or F11 can exit this mode.
Surprisingly, under about:preferences#permissionsData , there is no setting there to stop sites from being able to force Firefox into full-screen mode, either!
I think a new "Full screen" permission category should be added to that area of the settings, and by-default it should be set to "Default for all websites: [ Ask me to enter full-screen 🔻 ]", with additional choices of "Always allow" (the current behaviour; any site can make Firefox full-screen) or "Never allow" (where users would have to click the permissions icon in the left of the URL bar, and enable it there for a site; similar to how it works when requests to send notifications are set to "block new requests").
The current behaviour is silly: any site can force Firefox into full-screen, leaving some users then stuck.
