You have a fully functioning tool. The icon will turn blue when it detects a video. Click it, choose the quality, and download. When the "Pro" popup appears, ignore it or click "Later."
– Any website claiming to provide free license keys for the paid app is likely: video downloadhelper firefox license key
Without a paid license, many videos (especially DASH streams) now include a large QR code watermark covering a significant portion of the video. You have a fully functioning tool