Firefox is getting a feature from the Tor Browser called letterboxing, making it more difficult for advertisers to see your window size (via ZDNet).
[Mozilla Launches Firefox Quantum for Mac, Twice as Fast and More Customizable]
Letterboxing
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.
[A Tor Browser Might Not Be Your Best Solution for Internet Privacy]
Tor browser actually IS a stripped version of Firefox so you could see this a mile away. What you fail to mention is that once you change the size of a window in Tor (never mind other browsers where anybody can track you with simple java scripts & other browser exploits) advertisers are the LEAST of your problems – the bad guys AND the Feds are all over tracking you if a window is changed.
Again – VPN. Say it with me….VPN plus Tor is the best solution; and of course NEVER have javascript enabled as any kid can hack your Mac thru javascript. If a site requires it, make sure it’s a safe one “https” etc. and then disable it afterwords. d:D