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

This is the best plugin to download from Youtube. A lot of formats, works really well - please keep updating!

Working Perfect for 8 years

If i try download a movie where is already only one file "audio and video, like in 720p usually" it extracts the audio, save this file and delete the movie one, and keeps the audio file only, not the video, if the quality i choose it's in separated files "audio and video, like 480p" works nice.

很好！好多时候总是想看一下视频来缓解精神！

Excellent

It is the best of similar add-ons.

but

1. WebEx version does not allow [ . _ in filenames.
cf. [17_09.16] -> -17-09-16-

2. I use mkv as the combined file.
(ffmpeg -i %audio -i %video -acodec copy -vcodec copy %output.mkv)
WebEx version writes the filename extension twice.
cf. test.mkv.mkv

Why aren't the settings accessible through the firefox extension manager? Why only through clicking through the toolbar button?

Also, what is the correct format for 'ffmpeg path'?
To the executable? (yes, through trial & error)
To bin dir?
To overall ffmpeg dir?

What is the correct format for 'save merged files in'?
Quotes surrounding the dir or no? (no, more trial & error)
'\' at the end or no? (also no)
Why only 'merged files'? How can I set the save directory for all downloads?

'Consider OPUS' seems to fetch OPUS for MP4 grabs as-well even though the option makes it sound like it's for WEBM fetches only.

Where have the options to remove WEBM, OGG etc options from the download panel gone?

This add-on is much far better than other extensions.

I loved your old version of this addon before web extensions. It worked perfect. I installed the native client and have already had ffmpeg install and when I click on a youtube video, any video i get this message "cant convert undefined to object". I am using kubunto linux with firefox. I also installed you media converter and muxer for web extensions and it works great. What am I doing wrong. Hate to lose this addon. Please help.