The instructions say:
1. After installation, go to the options page and
-> 1. Install the native client (This is a small NodeJS client written by @andy_portmen)
-> 2. Install the FFmpeg executable
2. FFmpeg integration is required if high resolution video or audio files are preferred.
My questions are:
Where do I find the options page?
What is FFmpeg integration and how is it done?

Just as with the original, it is unclear from the configuration settings, what "you will get" upon choosing a download: a video-only file, an audio-only file, or a video with audio.

With the original, I know have it configured so that downloading a YouTube video delivers an mp4 video+audio and an mp3 audio-only. This is perfect, though I have not the foggiest which magical combination of unclearly worded settings gave me this wonder.

With the new WebFail version, I can not get the same result (video+sound, and audio). But worse, whatever setting I try, it downloads a video fail, then extracts an audio file, then deletes the video file under my eyes.

does what it claims,unlike others..

I downloaded and installed the extension. The instructions said to download an additional file from the options page. What options page? Where am I supposed to find it? There are no options given on the Firefox Add-on page. The old add-on worked great, this one doesn't work at all without an external file I can't seem to find. Also, I see lots of questions on this page, and no response whatsoever from the developer. So . . . the Add-on doesn't work and the developer doesn't respond. Why in the world would anyone want to deal with this nonsense?

Easy access, quick to learn, always works.

I'm glad that a new non-legacy version of this extension is available. I got everything to work, including adding the path to ffmpeg. Everything works like in the previous add-on. However, there's only 1 thing this one is missing: hiding filetypes of the available video downloads. I never use 3gp or WebM filetypes. I was able to hide these in the old add-on. Now these add extra "cluster" to the filetypes to download inside the video. Not a deal breaker, just extra annoyance over the old one.

The new version works good. But I can't find the download button which is the arrow button beneath the youtube vedio. there is just hollow space but it works anyway when I click hollow space.

Is there any way to set download directory before I download video? I try to set directory on setting but it doesn't work.

Liked the old version better. was much easier and it used to download both video and audio files when I click. can't find how to get it work this was again no matter how hard i try.

Best addon when it comes to downloading YouTube videos. However, it requires you to download external files to make it fully functional.

:(

This is very useful addon for me.
thanks

Cannot detect url_encoded_fmt_stream_map or adaptive_fmts.
Trying to download 3xbVuuxnJbg.
Hope You can fix it

Excellent add-on, and compatible with the new Firefox 57+ technology too... I agree with Rahoul, this is the last YouTube downloader you will try, there is no need to continue searching. I can't realize living without this gem. :-)

Tried a dozen of YT downloaders for Firefox. There was just one that was close to this one, but not close enough.

Tried this on two pcs (same os & FF version). One works fine, the other downloads the two streams but is then unable to find the downloaded streams! Rechecked the limited settings, reinstalled .. zilch. Only other difference - on the one that works I got a firewall exception request which I allowed. No alert on the other one. Suggestions anyone???? Short of manually merging the streams using ffmpeg.
The support page has several posts but no replies from the author.

Your search for the more apt youtube downloading tool ends here :)

Usado en Firefox Developer Edition 57.063 (64-bit).
Cuando se elige una calidad de bajada en el Download Panel, al terminar de bajar el archivo desaparece, no guarda nada. En cambio hay que hacer "Guardar enlace como", elegir dónde guardar y entonces sí lo baja y guarda. En versiones anteriores se podía usar el procedimiento descrito primero.

After looking at numerous other Firefox57-compatible video download tools, this is the first I've seen that accesses both audio and video streams and combines them into one file. It uses the well-known FFmpeg codecs, which needs to be installed and location inserted in the options dialog.

Bello