When you browse to a website, your browser passes info about itself to the server hosting that site. This info is intended to help the server provide the best rendering code for your browser. This is called your User Agent.
However, Google is using it here to identify Firefox users, and is apparently choosing to lump them all in a box called "adblock users" instead of trying to identify an ad blocker more accurately.
As a link to a different website, at least. This one links to imgflip