Correction NO Niggles, just Great Rated 5 out of 5 stars

The Niggle I mention below, saying that this Add-On does not provide links to the MP4 on YT is incorrect. The download link is there if the mp4 is there to download.

So this add-on is just great.

This review is for a previous version of the add-on (1.15.1-signed). 

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 (1.14.1-signed).