I am using as shown below, to display videos from youtube in my page>
Notice that Mozilla does not support Adobe Flash anymore and so when the browser gets the "application/x-sockwave-flash" it automatically downloads it since it does not know what to do with it.
Instead you should you use the YouTube share option (Under the video choose "share" andthen choose "embed") which gives you the correct way to embed it into an "iframe" for all browsers (Uses HTML5).
In your case the right url to use in the "iframe" is https://www.youtube.com/embed/DNLh8p3aUzQ