Developer seems to believe they need a service to convert something. YouTube sends split audio/video streams now. If you want the original video, you'll need to download both and convert them yourself. Of course, the developer could have done this on your local machine with ffmpeg, but decided to do it on a server instead and charge for it. The developer does not allow you to download the HD streams yourself so you can combine them locally.

Use this addon for now: https://addons.mozilla.org/en-US/firefox/addon/youtube-download-hd-download/

Every time I tried to use this extension I got the following quoted message pop up right below the video.
"Youtube switched to separate audio/video streaming technology for higher quality formats and no longer provides direct 720p (Since 24 September 2018)/1080p/mp3 streams. Converting these streams to downloadable formats for you needs powerful server infrastructure, therefore 1080p/mp3 downloading has been limited to PRO users. Read more at - https://videodroid.org/pro_upgrade.html. FREE users can download all formats in CACHE where some PRO users fetched the video/audio in past 24 hours, to test the feature you can try downloading popular videos."
I have read this 3 or 4 times and I am still not sure if it's an excuse or if they are asking me to buy the "pro version" Does anybody know of a good youtube mp3 converter that works as they say it does? or are all these extension creators all just out to make a quick buck? Sorry guys/gals give this one a pass unless you want to spend the money on a pro version. It does NOT do what they say it does.

Works great!

não consegui baixar

Love this apphavent had any issues with it I always get my fav music and videos for use off line

Great addon.

I've been using this add-on for a while and it is excellent. I can use it easily, its reliable - I totally recommend it!

Outstanding! This extension has cured all of my video download headaches. In my location there's only one choice for broadband and speeds are very inconsistent so downloading video to watch offline is a must. I became a Pro member to get access to higher quality video downloads and have no regrets. If you're using this extension, make a donation to the developer, who has created one of the best FF extensions ever!

Potrzeba zapłacić by działało.

Un Addon gratuit bien pratique pour télécharger des vidéos ou des audios que l'on peut ensuite convertir en mp3 (via vlc), bref un must.

I use this video downloader since years and I am very happy with it. Its well worth a donation for the pro-version. Regularly updated. Usually all problems solved within short time. Very cool!!!

have to pay