If you have a lot of stalled downloads like this, and if they're actually important to you, then you'll make the effort to do what I'm about to describe. It will be worth it to you & you'll be careful about doing it. Take your time. It's mildly tedious. But like I say, if you're sufficiently motivated to get those downloads completed, you'll do this even if it's not the ideal thing to do. This isn't an ideal world & sometimes you have to let go of certain lofty ideals & just git'r'done.
Make note of the web pages from which you were downloading videos. You could do this in a simple text file using Notepad or something analogous. If all the windows are still open, just copy/paste the page URLs into your text file. If they aren't all still open, there's Plan B. In Firefox, on the Downloads page, there is a popup context menu on each entry in the list showing "Go To Download Page." This can be a big help. Don't clear the entries from this page. If you're worried you might lose the contents of the download list, you can use the "Copy Download Link" entry in the popup context menu to save the download URLs off to your text file. If there are no entries on the Firefox downloads page, as may be the case for certain downloads using the CoApp, then you can get the information out of VDH one by one for your in-progress downloads. Tedious? Yes. But like I say, if you're actually motivated to get this accomplished instead of just complaining that it's not easy, then you'll do it. When you click the blue button & VDH shows you the list of in-progress downloads, hover your mouse over each entry. On the right end of the entry, a little box with ... and an arrow will appear. Click that. Another menu appears. One of the entries on this menu says "Details." Click that. You get a list of information concerning your download. Look at the one labeled "referrer." It shows the URL of the page from which you are downloading a video. That URL can be highlighted & copied to the clipboard using standard operating system techniques. Just copy/paste the URL into your text file. Once you've collected all the URLs, cancel everything & close all browser windows.
Now relaunch the browser & revisit all the pages whose URLs you collected above. If the browser Downloads page still contains the entries for your downloads, you can use those entries to revisit the pages & you won't need your text file. You may need to use a mixture of the Downloads entries & things you saved to your text file. If you're really a geek, you can turn the text file into a script & semi-automate re-opening the pages.
Or you can just ignore me & continue to complain about how things aren't the way you'd like.