Recommended. NO Malware. Beware of Fakes. Rated 5 out of 5 stars

Simpler than the rest and does the business.

And NO, this add-on does NOT add any malware or adverts.
However there is at least one other site pretending to offer this Add-On but instead (I presume) offering malware, e.g.
http://www.fnvfox.com/thankyou.html
So make sure you download/install this Add On from the official mozzilla.org page ADD-ONS page.

However, one niggle...I thought that YT offered a low resolution mp4 file (for all the ipod users perhaps). There seems to be no option to download an mp4 using this Add-on.

This review is for a previous version of the add-on (1.14).