Screen sharing with Firefox on Linux

During an event today (23 Oct 2021), I successfully shared my screen once. But the second time I had forgotten to open the document to share before clicking share screen. So, instinctively, I pressed “ESC”. I totally lost control of my mouse. I needed to log in from another computer to kill Firefox. When I tried again, Jitsi appeared to “remember” the errant “ESC” mode and I needed to kill Firefox again and I gave up on sharing my screen.

How does one exit from “ESC” mode in screen sharing in Firefox so that normality can be restored?

I run the Debian Firefox in Buster: 78.15.0esr-1~deb10u1