Firefox is getting a feature from the Tor Browser called letterboxing, making it more difficult for advertisers to see your window size (via ZDNet).
It’s possible for advertising tools to figure out the size of your browser window. This helps them with creating user profiles and tracking you as you resize the window and move to new browser tabs and URLs.
Letterboxing adds gray spaces to the sides of a web page when you resize the browser window. This will hide the true size of the browser’s width and height. It tricks the advertising tools which will collect the wrong browser size. Letterboxing was first introduced to the Tor Browser in 2015.
The feature is currently available in Firefox Nightly and will roll out to all users when Firefox 67 is released in May.