At a conference today 25 Sep 2021, I was unable to share my screen with the Debian Firefox browser. I was using the latest security patch release 78.14.0esr-1~deb10u1.
When I shared the screen, it looked OK in my browser: I saw my shared screen in the Jitsi conference. However, other participants reported that they saw me in the camera and the screen share did not work.
I reconnected with chromium and screen sharing worked correctly.