Hmmm, I've never had that problem downloading from YouTube with that extension.
I just tried copying and pasting the URL to the video into an online video VideoDownloader and it reported a bad video ID, but this site (KEEPVID) doesn't seem to have a problem.
http://lax-v222.lax.youtube.com/get_...id=fKbekY6srtI - this link here is a direct link to download the YouTube clip you want using KeepVid. NOTE It will download a file called "get_video.flv". What you need to do is rename that file to something more relevant and you will also need to download a FLV video player or codec.