CatBlock was an extension based on AdBlock that replaced ads with either cat pictures or pictures of your choice from Flickr. It was originally introduced as part of AdBlock as an April Fools' joke in 2012. You all loved it so much that we kept it as a standalone extension until October 2014, when it was discontinued. It is now available for free for anyone to use. The source code is open-source under the same license as AdBlock. You can get it from catblock.getadblock.com.
When you are installing CatBlock, if you are asked for the email address you are using to support AdBlock, simply enter: firstname.lastname@example.org.
CatBlock is not supported anymore, but if you're having issues, the following suggestions may help:
1. First check on http://nytimes.com that you see cats instead of ads. There should definitely be some cats showing.