To get Youtube videos, I use an add-on in Firefox called DownloadHelper.
The add-on puts an icon in the toolbar of the browser. To use it, you go to the Youtube page with the video you want, and the icon will start rotating, indicating that it can download something here. Right click on the icon, go to Media…choose which version you want of the video, then click download. That’s it.
You’ll get either an mp4 or flv version of the video, depending on what you told the add-on to do, which can be used however you wish.