I decided to give it a try to see if fingerprinting protection was enabled in the browser, and if turning it off would resolve the screen size issue. Here is what I did: Load about: config in the Firefox address bar. Confirm that you will be careful if the warning message is displayed. Search for privacy.resistFingerprinting. If the preference is set to True, the extra fingerprinting protection is enabled, if it is set to False, it is disabled. If True, set it to False and restart Firefox. The preference was set to True on the system. I changed its status to False and restarted Firefox. Firefox did remember the correct window dimensions this time when I restarted it, and further tests showed that the browser remembered the window dimensions each time I started it. So, if you run into that issue you may want to check the preference to see if it is responsible for that. It is up to you then to keep the protection enabled and live with the rounded window dimensions, or turn it off and have Firefox remember the right window dimensions each time.