Unfortunately, since Firefox 57 and beyond Mozilla no longer supports XPCOM type of add-ons, such as the old version of AVD.
We continue to work on migration AVD to a new platform and returning the habitual appearance as much as it possible in new Firefox versions, but it still may not support some features that the old AVD is used to.
You still 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/
More about new AVD 3.0 you can read in our article here: https://support.ant.com/hc/en-us/articles/115005568983-About-the-new-Ant-Video-Downloader-3-0