Yeah, you're not the only one hitting this problem. For your file in use issue, you
might be able to do something less drastic than closing all Firefox windows. Reload the
extension instead. How? Click this:
https://groups.google.com/g/video-downloadhelper-q-and-a/c/BzPLK2YyL-s
Make sure you expand it so the full text of every post in that thread is visible. Read
the first 2 paragraphs. Then look for (Find in Page) "reload the extension" within the
text of that web page. That will give you a link to click. Click it, read what you
find, & follow the advice. See if that unlocks those files you want to delete. Come
back & post about how it goes.
You might get some relief by doing the July 17 step. What's that? I've just done a
search of this forum using "july 17" as my search key. There's a bunch of hits. You
should get meaningful advice from one of the earlier search hits.