Apple has been doing a lot of security updates recently for Mac operating systems. To check to see if your screen sharing is enabled correctly on your Mac, check your privacy settings. Click the Apple icon in the upper left hand corner of the screen > System Preferences > Security & Privacy. Click the Privacy tab at the top of the dialog box. Scroll down on the left hand side of the screen to Screen Recording. Check the box next to Chrome and Firefox. This should resolve the issue. You can also do the same for your Camera and Microphone.