Warning: Use these scripts at your own risk. Use of Greasemonkey scripts are not supported by Meebo.
“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.” - DiveIntoGreasemonkey.com
You will need Firefox to use Greasemonkey
I suggest reading up on how to program in JavaScript/HTML/CSS if you don't already know how. A lot can be learned from http://diveintogreasemonkey.org/. It has tips and tutorials on how to make your own scripts. You will have to get familiar with Meebo's scripts to do anything. An excellent way to look into the code is the Firebug extension for Firefox. Although this does have a bit of a learning curve, Firebug allows you to look deep into the source of meebo, allowing you to get a better picture of what is going on.