If you are using macOS (10.15/Catalina) and screen sharing doesn't work on Firefox, following these steps might solve the issue.
1) When trying to start screen sharing, the following dialog box will be displayed (screen sharing will fail)
2) After screen sharing fails once, a dialog screen will appear, where you can allow access from “Preferences”> “Security & Privacy”> “General”.
3) If you allow access and try starting screen sharing again, delivery will fail,
but the dialog screen mentioned in step 1 will now display an "Open" button. Click "Open" to reload the page.
Once you reload the page, you'll be able to start a Live with screen sharing.