- Know and understand the security-related implications of making this change
- In the Tor Browser address bar, type in
about:configand press enter
- Search for
- Set the option to
- Restart the browser for changes to take effect
Tor Browser 9.0 is configured to use letterboxing by default. The technique adds white margins to the browser's frame to let users resize the browser window while still prevent fingerprinting using screen dimensions.
It will mask the window’s actual dimensions by adding space to width and height at multiples of 200px and 100px while resizing. So the gray space will be added at the top, bottom, left, or right of the current web page.
Since advertising codes listen to window resize events to gather this info, Firefox 67 will provide a generic dimension to such trackers and bring back the window to its actual size milliseconds later. So basically, letterboxing delays the loading of actual page content on the newly-resized window; long enough to trick the tracker codes or scripts to read the incorrect window dimensions.
Letterboxing isn’t a new technique since it was originally developed by Tor Browser in 2015 coming from experiments that the developers of the Tor browser conducted and is part of the Tor Uplift project that introduces improvements in the Tor browser to Firefox (Tor browser is based on Firefox code).