I’ve bee having fun using Grease Monkey to mess around with DHTML of websites in Firefox. My first shot was a very simple Yahoo Cleanser user script that removes the ads on the main yahoo page. The possibilites are endless given the use XMLHTTPRequest , TreeWalkers and an array of other mozilla specific features. There are already hundreds of user scripts available for download.
To Install navigate to the script and choose Install User Script from the Tools menu, and Click OK. Next time you visit yahoo.com it should be nicely cleansed of ads.