It used to work great, but not now. Now squeezing users for money.
720p is now "HD" and requires paid Pro feature. No mp3 downloads.
SAD!

It works, but practically only for 720p video. Does the job if you really want a downloader browser extension, but it's really limiting otherwise. I'm not bothered that mp3 is limited to a paid feature, it's easy enough to convert video files to audio only. What does bother me is the lack of 1080p or higher downloads.
Their excuse about needing "powerful server infrastructure" for 1080p+ downloads is a weird one, considering this is all done locally. You could convert to mp3 locally as well, but that's whatever.

All in all, it works. But you're better off just using software that lets you download as you please.

The best online YouTube video downloader. I have used this add-on at least ten years. You can download 1080p videos or convert videos to mp3 high bitrate files. If there is a technical problem, one gets a good e-mail support. I always get a reply if there is a problem with the add-on.

Lame. Doesn't actually let you download MP3s unless you pay for "pro". Not worth!

Es genial puedo descargar el video que quiera y en cualquier formato, buen trabajo.

I remember when this ACTUALLY WORKED FOR FREE, NOW WE HAVE TO PAY FOR THIS "PRO" TRASH AND I DON'T SEE AN ICON ANYWAY SO THIS IS A GREEDY HUNK OF TRASH!!!!!!. Why this is greedy is because this appears as fully free unless you scroll ALL THE WAY DOWN to the place where it says "use pro for, this is now me speaking for a bit, EVERY MAJOR FEATURE, LIKE MP3 DOWNLOADING AND 1080P DOWNLOADING". Never using this again.

This is a GREAT add-on. I'm an old Co-Lo Data Center tech & I almost never ever buy Xtra little software. But this YouTube DownLoader is absolutely one of the most useful and Xcellent working add-ons you could have for FireFox. I use it several times a day almost every day / all day Long.

Working to slow

Requires you to pay before it will let you download anything. You get a "Download" button on Youtube but this "download button" actually forwards you to a page that requires you to pay at least $10 to use the extension.

yt-dlp is free people. Use that.