AdBlock is a content filtering and ad blocking extension for the Google Chrome, Apple Safari, Mozilla Firefox, and Opera web browsers. AdBlock allows users to prevent page elements, such as (and mainly) advertisements, from being displayed. It is free to download and use, but users can donate any amount they want to the developers. As of July 2014, AdBlock is the most popular Google Chrome extension, with over 40,000,000 users, and the most popular Safari extension.