YouTube recently improved several settings, which has affected many downloading tools. YouTube does not provide any download option to its videos, but users can download YouTube videos in all supported formats (like MP4 HD, 3GP, MP4, FLV) using several existing tools like browser add-ons. This recent change has affected many browser based youtube downloaders – and users were unable to download videos from youtube.
Thanks to active developers, as many of them have now updated their browser add-on / extension to work with the latest settings. If you are looking for a working tool for Firefox – try the popular greasemonkey script "Download YouTube Videos as MP4" or the Easy YouTube Video Downloader add-on.