Currently, we're unable to reproduce the issue, but we suspect it may be caused when updating from version 4.9 to 4.10, and then to 4.11-4.12.

The problem may be in the data left from version 4.9. Unlike Chromium-based browsers, Firefox does not delete extension storage when you uninstall it, even in debugging mode.

Try resetting Ant Video downloader, go to Settings - Reset, click the WIPE OUT AND RESET button. This will delete any data in the extension storage, and reload the extension.

Let us know in the support if this helps you solve the issue.

Thank you, and sorry for the inconvenience.