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.
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.
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.