Just updated to Firefox Quantum and my old standby Video DownloadHelper was relegated to legacy status until a new version is available. Clicking on "Find a replacement" led me to Video Downloader Professional . Installed quickly but when I clicked on Options in the Extensions list I got "File not found: Firefox can't find the file at moz-extension://bc627172-939c-4d5c-9189-4105e6f68299/options.html ". Clicking on the homepage as listed goes to a page trying to sell 3 versions of Video Downloader Ultimate from Link64 GmbH (which is a stand-alone windows program) and there is no mention of the Firefox Add-on. Not seeing any videos most of the time enev on YouTube.
Uninstalling now!