Firefox is set to start blocking cookies from third-party ad networks by default, thanks to a patch submitted by Stanford law student and online privacy activist Jonathan Mayer. The patch is slated for distribution in release 22 of the popular browser, and mimics the behavior of Apple’s Safari, allowing sites that you’ve actually visited (first parties) to set cookies on your system, but blocking cookies from third parties like advertising networks unless they already have one on your machine. Firefox already supports the Do Not Track header, which has the effect of asking advertisers not to track your browsing around the web, but Mayer’s patch goes a step further, adding a default setting that refuses unwelcome third-party cookies altogether.

News of the patch is already drawing the ire of the ad industry

A cookie is a digital identifier that allows a site to store information about you pseudonymously, like the contents of your online shopping cart. By setting cookies, a third-party ad network can track users’ browsing activity across all the sites on which it serves ads, forming the basis for what’s called Online Behavioral Advertising (OBA), or the selling of targeted ads to individual users.

News of the patch is already drawing the ire of the ad industry, with Interactive Advertising Bureau VP Mike Zaneis calling it a "nuclear first strike." But given Firefox’s declining share of the browser market and the fact that Safari has been blocking third-party cookies on the desktop and mobile for ten years, the overall effect on advertisers’ bottom lines isn’t likely to be quite the catastrophe Zaneis is evoking.