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.

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?

:(

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.