Robert in CA wrote:

That sounds beyond me like the FFMPEG.
I do have Adobe Flash installed but I
can't use that to create movies only to
watch them is that not correct?

Robert

I installed it, in Windows 10 in a VM, with a
Firefox 67 or so.

https://en.wikipedia.org/wiki/Video_DownloadHelper

It asks for a "Companion App".
This is what it expects you to download.

https://github.com/mi-g/vdhcoapp

So what happens is:

1) Download and install Video_DownloadHelper
2) Try to snag your first Youtube video.
3) It gets part way through and asks for Companion App.
And that file is not coming from the Mozilla site
as far as I know.
4) Now, when you download a video, it's going to save it,
put a watermark on top of it, and ask you to pay to
remove the watermark.

*******

I think the problem here, is John is using the old version
which is .xpi based (works in Firefox 52ESR perhaps).

The newer version is for WebExtensions on the newer browsers.
And it got broken into two or more pieces.

Just a guess.

Maybe John can elaborate on how well the .xpi version works.

Paul