Downloads a Document File (?!!) Rated 1 out of 5 stars

I don't know why I'm experiencing difficulty. The add-on appears to have been added successfully, as I see the download button below the player, but when I click on it and choose the mp4 file I want (either of them that are available), it is saving a Document file (??) called videoplayback rather than an mp4 file. Can anyone help and tell me why this is happening? Thanks in advance for your help!

This review is for a previous version of the add-on (1.7.15). 

Get the latest version (1.7.16): https://addons.mozilla.org/en-US/firefox/addon/download-youtube/versions/