Need to download videos from YouTube? This Firefox extension will let you do so with many different (including HD) options to choose from. The extension also lets you download DASH video and audio streams. Please be aware that DASH files do not include either audio or video streams. You need to download one separate DASH audio and one DASH video file and then combine (MUX) them together in order to get a normal media.
Here are the resolutions this script supports:
Want to learn more? You can pick up the YouTube Downloader Lite add-on for Firefox from the Firefox Add-ons website.