Taken from Wikipedia~ Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to most HTML-based web pages. As Greasemonkey scripts are persistent, the changes made to the web pages are executed every time the page is opened, making them effectively permanent for the user running the script.
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Once you have installed Greasemonkey then you can go wild, available tabs at the moment are Google Reader, Facebook, Twitter, Netvibes, Plurk, ReadBurner, his own version of a Techmeme tab inside of FriendFeed. And also scripts to add tabs for TechCrunch and CrunchGear. Happy days! So far I have just added Google Reader as I use that to keep up with all the sites that I follow and Plurk. I mentioned Plurk yesterday and although it's creating discussion at the moment I am still not convinced on it's longevity. I've given it the benefit of any doubts with a tab of it's own so I am not going to abandon you yet Plurk. I'll end by directing you to this post from Duncan which goes into more detail on issues that may be experienced!