A few years ago, Firefox was quickly becoming irrelevant in an online world where Google’s Chrome, Apple’s Safari, and Microsoft’s IE dominated the landscape. But then Mozilla pivoted and began building privacy features into Firefox that other browsers lacked. That privacy pivot paid off, and Firefox quickly gained ardent users who wanted to take back control of their online privacy.

Now, Mozilla is working on another privacy feature that will make it even harder for advertisers to identify and track you around the web. As ZDNet reports, called letterboxing and coming in Firefox 67, which is scheduled to be released this May, the feature is a new anti-fingerprinting technique that blocks online ad trackers from reading the true dimensions of your browser window and using that information to identify you.

“Wait, what?” you might be saying. But yes, data firms and advertisers can identify your unique profile and track you around the web based on the size of your browser window and any time you alter its dimensions. It’s just one trick in the “fingerprinting” arsenal companies use to identify who you are based on your computer’s characteristics. Other fingerprinting tricks include identifying you based on your computer’s installed fonts and the extensions you use in your browser.

Firefox’s letterboxing will mean these companies have one less way to ID you. As ZDNet explains:

The general idea is that “letterboxing” will mask the window’s real dimensions by keeping the window width and height at multiples of 200px and 100px during the resize operation–generating the same window dimensions for all users–and then adding a “gray space” at the top, bottom, left, or right of the current page.

The advertising code, which listens to window resize events, then reads the generic dimensions, sends the data to its server, and only after does Firefox remove the “gray spaces” using a smooth animation a few milliseconds later.

In other words, letterboxing delays filling the newly-resized browser window with the actual page content long enough to trick the advertising code into reading incorrect window dimensions.

Letterboxing originated in the Tor Browser, and it’s great to see that such security features are making their way into more consumer-level and user-friendly browsers in a day and age when companies are trying to find out all they can about our online activities.

Update: a Mozilla spokesperson reached out with the following comment confirming Letterboxing is being worked on, but the company is not yet sure if the feature will be available to general users. According to the spokesperson:

“At Mozilla we work in the open and we are still experimenting with Letterboxing to get more feedback. Currently, the Letterboxing feature is disabled by default. We have not yet determined if or when this feature will be available to general users.”