You must have heard about “YouTube Instant”, similar to Google Instant, created by talented developer Feross Aboukhadijeh (who got job offer from YouTube CEO!). Unfortunately, to use this “YouTube Instant” you need to visit another webpage as it is not integrated with the search box on YouTube.com. Hence, it becomes an experimental tool only.
To solve this problem, Stonos has created a Greasemonkey script which integrates the instant search feature with the search box on YouTube.com homepage. You can install this user script on Firefox with the help of Greasemonkey add-on, and see the results as you type the query.
Install this script from here. Currently, this script works only on homepage of YouTube.com.