People don't be fooled by this author. This is not a Youtube search plugin that you put as a firefox search engine provider, it is more like a shady version that is like those infamous spyware toolbars. So do yourself a favor and just visit the actual YouTube website and then add the search engine from there when you click on Firefox's search engine icon.
This looks like it should be what I wanted - it puts a small icon on the bookmarks toolbar, clicking it opens a search box and the video plays in a small window top right.. Sadly, all that happens in Firefox 13 is the YouTube window draws and the video plays, but the screen is black and the video can't be seen. One to watch if this glitch can be fixed.