You will find this button on the Mozilla add-ons for Firefox page
It is possible that Firefox could block the installation. You will see a yellow bar at the top of the page, telling you that the installation is blocked. You must click on the Allow button, at the right of that yellow bar, as you can see in the image below.
Click on the Install now button to start the setup.
The setup of the video downloader is completed, click on the Restart Firefox button.
You can now use Ant.com Video Downloader, it's very easy:
1- Play the video you want to download from any tube site.
2- Click on the Download button, located at the bottom right of your Firefox browser.