Rated 5 out of 5 stars
It's broken again in the latest Aurora and nightlies, even after 4.9.12. In fact no video downloaders work right that I can find.
Don't know what the firefox developers are doing except discouraging use of the newer versions.
Edit: after creating a temporary new profile your latest version does appear to work with the new download manager in UX nightly. But with "browser.download.useToolkitUI" set to true it doesn't, the box just pops up briefly then closes. Hope that helps.
Can you contact support at downloadhelper.net with details on your exact configuration ?