Mozilla made a surprise announcement yesterday: Firefox will soon block web trackers by default. This is a privacy and performance gain that Google is going to have a hard time matching in Chrome.

Let me explain.
I’ve been hoping Firefox would start blocking trackers by default for a while now, ever since the company added Tracking Protection to Firefox 42’s private browsing mode in November 2015. With the release of Firefox 57 (Firefox Quantum) in November 2017, Mozilla added an option to enable Tracking Protection outside of private browsing, but to this day it still isn’t on by default. It is in Firefox for iOS, however, and Firefox for Android is expected to follow suit, but on desktop, Tracking Protection remains something users must turn on manually.

While I still wish Mozilla would have simply turned on Tracking Protection by default in Firefox for all platforms, my understanding is that simply too many sites break for this to be a viable strategy, at least right now. And yet, we learned yesterday that the company is building three tracker-blocking features into Firefox for desktop, specifically targeting trackers that slow down page loads (to improve page load performance), trackers that work across sites (strip cookies and block storage access from third-party tracking content), and other trackers that fingerprint users or mine cryptocurrencies.

The second one, removing cross-site tracking, is the real game-changer. This is what I mean when I say Firefox is hitting Chrome in the ads. I’m not talking about straight-up blocking ads. Sure, Chrome has started blocking some ads by default, but its implementation is by no means a full-blown ad blocker. Mozilla’s declaration of war on trackers also isn’t going to block ads directly. But it is going to significantly hinder how advertisers can target users. Blocking cross-site tracking by default is a massive win for users.

Chrome can’t respond to Firefox with the same functionality without some serious soul-searching. Google’s parent company Alphabet derives most of its revenue from ads, and cross-site tracking is critical to thousands of advertisers.

Google will likely continue cracking down on “bad ads” in Chrome. That might look good on the surface, but in terms of performance and privacy, Mozilla’s new Firefox strategy will be much more effective. Mozilla doesn’t owe advertisers anything.

As I’ve argued before, Chrome and Firefox are bringing back the browser wars. While Google beat everyone to the punch with its selective ad blocker, Mozilla is hitting back right where it hurts.