The most annoying thing for me when it comes to Firefox+Videos is it's full screen confirmation message. When ever you try making a YouTube video full screen, Firefox will show the following message-

And it is after the video has been made full screen!

You can click on the "Remember decision for" to make Firefox not show this message for further YouTube videos. But that doesn't stop it from showing the same message for other video sites.

If you want to disable this message permanently in Firefox, here's a little trick. Open a new tab and type in URL "about:config". Click on the "I'll be careful, I promise" button if shown to go to the page. Now search and find "full-screen-api.approval-required" preference and set its value to false.

That's it you will never have to see the confirmation question ever again.