You can install Ant Video downloader from official Firefox Add-ons website, also known as AMO (from its domain name addons.mozilla.org): https://addons.mozilla.org/en-US/firefox/addon/video-downloader-player/
You will be also forwarded to this page in case if select Install for Firefox button at the AVD page on ant.com website here:
Click on + Add to Firefox button and then Add button in the popup that appears to confirm the permissions.
You can install one of previous AVD versions from Mozilla website here:
https://addons.mozilla.org/en-US/firefox/addon/video-downloader-player/versions/
- or from ant.com archive here:
https://archive.ant.com/firefox/
You can also save one of previous AVD versions from AMO or ant.com archive by right-clicking at desired AVD version link and selecting Save Link As... option and selecting saved XPI installer from your browser's File - Open File... menu.
Please note that you need to disable AVD automatic updates in case of planning to use the old version. 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.
Note that you can also install Ant Video downloader on a derived web browser (either based on Chromium or Mozilla Firefox) such as UC Browser, Waterfox, COMODO IceDragon.
After you installed the browser extension, you need to download and install the Native Application.