You can install old AVD version in Firefox 56 and below from Mozilla website here: https://addons.mozilla.org/en-US/firefox/addon/video-downloader-player/versions/

Please note that you need to disable AVD automatic updates in this case. You need to go to the Tools - Add-ons menu in Firefox, select Extensions tab, click on More link at Ant.com Video Downloader field and check Off option in the Automatic Updates row.