Mozilla really do need to remove this extension from the "recommended" section. Or even remove it altogether at this point.
It used to do exactly what it says on the tin, but nowadays it adds a large watermark in the corner of every video you download. If you want to remove the watermark you need to pay $28.50.
Paying almost $30 for a video downloader is ridiculous. If you already have this addon installed and you're reading this review, remove it immediately and find an alternative.