Tried a lot of these addons over the years, this is the best. Currently works really well at the time of review.

It adds a really obvious download button right alongside youtubes interactions buttons.

Supports downloading in a variety of formats, including ripping the audio out of the video file - very useful for me who was after old BBC Radio 4 transmissions that have been pulled by BBC, even though i've paid a license fee for 35 years!

Downloads really fast. Hope it will last. Provided it continues working for a month or so, i'll donate as it is well deserved.