The purpose of the message is to detect when sites trigger fullscreen since that can be used to phish you with a deceptive UI. But 99.9% of the time, it's just a video, so... (1) In a new tab, type or paste '''about:config''' in the address bar and press Enter/Return. Click the button promising to be careful or accepting the risk. (2) In the search box above the list, type or paste '''full''' and pause while the list is filtered (3) Double-click the '''full-screen-api.warning.delay''' preference and enter 0 (that's a zero) and click OK (4) Double-click the '''full-screen-api.warning.timeout''' preference and enter as short a time as you like, in milliseconds, and click OK. For example, 250 is a quarter of a second.