Mozilla is scheduled to add a new user anti-fingerprinting technique to Firefox with the release of version 67, scheduled for mid-May this year.
Advertising networks often sniff certain browser features, such as the window size to create user profiles and track users as they resize their browser and move across new URLs and browser tabs.
Called "letterboxing," this new technique adds "gray spaces" to the sides of a web page when the user resizes the browser window.
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 window 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, and sends the data to its server. In other words, letterboxing tricks the advertising code into seeing the newly-resized browser window at incorrect dimensions. The browser window can have any shape and size, but the page content inside it is only displayed at certain preset dimensions and the rest is filled with a gray space. Read more