Adblock is a free extension (browser plugin) available for the Mozilla FireFox web browser. It enables you to block online advertisements from the domains which you specify.
Installing and running Adblock, which is free, improves your surfing experience by decreasing page load times, and making webpages less cluttered.
You can also put a stop to layer ads (typically unblockable), and any popups which may find a way around the FireFox popup blocker, by using this extension.
However beware that free content on the web is only available to you for nothing because of the advertising revenue which the content publisher receives when ads are shown to users. If you block these ads you are harming the content providers as they are loosing revenue.