When you toggle Firefox's full screen mode (F11 key), this message does not appear and you can't use ESC to return to regular mode.

I'm going to guess that what you're seeing is generated by the Flash player plugin or by particular Flash media. Does that sound right?

I don't know of a way to modify that behavior, but maybe someone else will have a solution.

Try to disable hardware acceleration.

• Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available"

If disabling hardware acceleration works then check if there is an update available for your graphics display driver.

• https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers

I think it is normal for the message to display for a brief time.

If it persists for the entire time the video plays, it could be because you have disabled the Flash data storage feature. You can turn that back on here:

http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager03.html

This video shows the result: http://www.youtube.com/watch?v=oVWbqxeyZjI

Completely removing the message requires actually changing the code of the Flash player software, which is not recommended for security reasons (whether you follow online instructions or download something from a stranger).

Thanks for your reply; I have posted more info re the prob

Another update: The message lasts for several seconds but obscures part of the picture for that time. Once I know to use the esc key to toggle out of the full screen mode I obviously don't need the reminder every time, and it becomes irritating. That's why I want to get rid of it.