Just to clarify, is this problem only in Firefox or is all of Windows affected? If it's just Firefox, does the shrinkage affect Firefox's user interface (e.g., size of tab and address bar text) or only the page content?

Did this happen after after your beta version updated? You can check if there is an update for your graphics display driver. *https://support.mozilla.org/kb/how-do-i-upgrade-my-graphics-drivers You can try to disable hardware acceleration in Firefox. *Tools > Options > Advanced > General > Browsing: "Use hardware acceleration when available" You need to close and restart Firefox after toggling this setting. *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes *https://support.mozilla.org/kb/upgrade-graphics-drivers-use-hardware-acceleration