You can use Firefox to download videos from YouTube. There's a free add-on that puts a download button under the video.
Here's the add-on:
Open that link in Firefox and click the green add to Firefox button. Then go to the YouTube page and look for the download button below the video player. Select one of the MP4 options when you click the button. If there are multiple MP4 options, the higher number means better quality video but also a larger file size.