I'm going to guess that you did not read the confirmation page you got when you installed
the latest beta of VDH for Firefox. You did install the beta. It is specifically
updated to cure many problems on YouTube. So if you're not running the latest beta, get
it. Where? Go to the home page of this forum. Look at the page. Really look at it.
Look at it as if you'd never seen it before. Specifically, look at the part at the top
that your eyes are used to skipping over. Read it. You should be able to figure things
out from there. While you're at it, you also need to be on the latest release of the
CoApp. You should find where to get that when you find where to get the latest beta of
the browser extension.
Since you didn't read the confirmation page, you can learn about what you missed. You
have to search this forum for a thread with the title, "How to download from YouTube."
That is the complete title of the thread. Don't settle for anything else. Within that
thread, look for the post dated July 17. That gives you instructions with screenshots of
what you most likely have not yet done. Once you have done that, your problems should be
cured.
Here's a demonstration. There is a filter at the top of the YouTube home page that
restricts the display to videos of resolution 4K. I clicked that, then picked the first
4K video it showed me. It happened to be this one:
https://www.youtube.com/watch?v=eQGYHTc51NE
As you can see in the attached image, VDH is quite happy to offer you a 4K variant for
download. Note that it has the Side Download icon on it. If you ever see VDH offering
you Side Download variants, those are the ones you should use. Always use Side Download
when it is offered.