I tried both the firefox app and the online UDL Helper downloader. Both worked but only up to 360 quality. Neither worked to download HD 720 or 1080 or anything better than 360 unfortunately.
Our developers have put in a lot of effort to fix existing issues and have released a new version of the extension. Please try reinstalling it. If the extension works well for you, we would be happy to receive your positive feedback!
Best regards, YouTube Downloader (UDL Helper) team!
