When I try to screenshare I can see the application being shared in the firefox window (the side of user who is sharing), but the other audience in the meeting aren’t seeing the content being shared. This works when shared from Chrome, but not from Firefox. Also Vice Versa, when I join a meeting via Firefox, I don’t see the screenshare icon on bottom left
It’s the latest version I downloaded today as most of users in our company are prohibited from using Chrome and so, use Firefox as default user.
We use Firefox 75 (64-bit) in Mac Mojave
Sorry to say this but Chrome/Chromium remains your best option for screen sharing 
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