Youtube.com NOTE: This only downloads in MP4 FORMAT.
Step 1: Getting Firefox If you do not already have it
Ok so if you are going to do this you will need to install Firefox: here
When you are done with that, on to step 2
Step 2: Launching Firefox, adding the tool
(Highlight this and copy it to the little "Paste" sign pictured below.)
Step 3: Go to a Video and test it out
here is a video to test it on please do not take it personally :