Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to web page content after or before the page is loaded in the browser (also known as augmented browsing ). The changes made to the web pages are executed every time the page is viewed, making them effectively permanent for the user running the script. Greasemonkey can be used for customizing page appearance, adding new functions to web pages (for example, embedding price comparisons within shopping sites), fixing rendering bugs, combining data from multiple web pages, and numerous other purposes. Keep Tube :— It allows you to download YouTube videos in High Definition (HD), High Quality (HQ) WebM files, Mp4 files, 3GP files, FLV files as well as converting them to Mp3 file format. You will get a transparent download button on the bottom right corner of all supported websites as well as a download link near the title of all YouTube videos! Get the script from