Greasemonkey is a Firefox extension that allows you to write scripts that alter the web pages you visit. You can use it to make a web site more readable or more usable. You can fix rendering bugs that the site owner can't be bothered to fix themselves. You can alter pages so they work better with assistive technologies that speak a web page out loud or convert it to Braille. You can even automatically retrieve data from other sites to make two sites more interconnected.
step 1. Download extension: go to url,
step 2. to test greasemonkey working, Download User scripts go to url
step 3. open youtube.com and look for "download". Now you can download from youtube site.
How to start: go to url
How to write Greasemonkey scripts: go to url