- Download and install Firefox Web Browser.
- Click: Tools --> Add-ons
- Select: Get Add-ons
- Search for Download
- Look for "Flash and Video Download" and install
This add-on not only works for downloading a local copy of a YouTube video, but it also recognizes other video from sites as well.
You will see a new button on the upper-right corner of the Firefox browser. When the add-on recognizes a downloadable video, this will change to indicate that the video currently being viewed is downloadable.
Clicking on the button will offer the different resolutions available as well.