Hi! For the demo page, do you get any error message? Or does the download seem to complete (download icon turns into a checkmark) but nothing is downloaded?
On Firefox for android, you need to open the add-on once in your browsing session BEFORE loading the video, then reloading the video / refreshing the page and trying again. And due to a bug in the the android version of firefox (only on android) that I have no influence over, the download can only start when you open the add-on from a non private window, so try opening it from a regular window instead.
