Adblock is an extension for the Mozilla Firefox web browser which can filter content based on source address. Filters can use simple wildcards such as * or Regular Expressions. When a web page is delivered, Adblock can be configured to hide or prevent downloading of elements such as images or iFrames. Filters can be modified from the Tools -> Adblock -> Preferences menu item in Firefox or quickly added by right-clicking on the undesired element and selecting the Adblock menu item. Extensions can be downloaded from https://addons.mozilla.org/extensions/. Adblock specifically is located at https://addons.mozilla.org/extensions/moreinfo.php?id=10.

A simple filter such as */ad/* would block websites that keep their banners in a directory called ad. Another example of a useful simple filter could be *doubleclick* to block ads from Doubleclick.

While not useful, the Regular Expression /.*(google|yimg)\.com.*\.(jpg|png|gif)/ would block all .jpg, .png, and .gif from Google and Yahoo. Regular Expressions can make for a very powerful and flexible ruleset. This one rule blocks multiple file extensions on multiple websites.