Firefox's built-in Tracking Protection uses Disconnect's blocklist to block content that tracks you, preventing the tracking from happening in the first place. As a side effect, it blocks some, but not all, online ads. The thought was that online advertising is still an integral part of the web's sustainability, and so blocking it entirely would be too much - but advertisements that nonconsensually track users are a privacy and security risk, and should be blocked.
Until we have a reasonable alternative to advertising for funding the web, I personally consider Firefox's Tracking Protection to be a reasonable middle ground that hopefully sends a strong signal to advertisers - online tracking is creepy, unwanted, and puts millions of people's privacy at risk.