From youtube, take the URL from the "input" field of your browser. Click the "Opem media" button, insert the URL
AND delete the "s" from https !
Set the size from 550 to 450, leave the 0 untouched.
In FF, it does not open in the edit area but when it's posted.
Try with this one and be happy! (from 45")
https://www.youtube.com/watch?v=7jWyuCQflG0&feature=player_detailpage