In the Firefox address bar type
about:config and hit enter, then click the I accept the risk button.
Then search for
full-screen-api.warning.timeout and set its value to
After this change the message is not displayed any more when switching a video to full-screen, but you still get the message when you move the cursor to the top edge of the screen.
If you want to disable the warning completely, you'd also have to change the value for
-1. This change prevents the warning from being displayed if you move the cursor to the top screen edge.
Please be reminded that doing this might be a security risk. Take a look at , the article provided in the comments under the question by Duncan X Simpson. This site shows how an attacker could use the full screen API to fake a website to gain access to the credentials of your online-accounts. You shouldn't "click the link" on that site unless you're able to figure out what's going on behind the scene; just read.
But as the warning pops up for only three seconds by default, one could miss the warning due to looking away from the screen at that moment for some reason. For real security the warning shouldn't time out but stay on the screen until one clicks to hide it (that would be work for the firefox team).
However, 100% security never exists and as always we have to weigh security against convenience, so if you disable the warning, keep your eyes open!