Please run firefox with G_DEBUG=fatal-criticals under gdb (firefox -g -d gdb) and attach the backtrace here, see:
https://fedoraproject.org/wiki/Debugging_guidelines_for_Mozilla_products#Application_crash

gtk_widget_get_window: assertion 'GTK_IS_WIDGET (widget)' failed"

usually means null pointer is passed to gtk_widget_get_window().

Martin, thanks for the info.

The "gtk_widget_get_window: assertion" looks harmless, there is no crash registered, but the message appears when I confirm/select the "microphone" permissions (I have no camera attached). I guess I'll buy a web camera and prepare a debug build before my next attempts :-) I'll keep you informed.

I have tried sharing a window from the ppc64le machine and it works = was seen correctly on the other machine in the Google Meet session (my x86 laptop).

In testing with a Bluejeans session, I see the same "gtk_widget_get_window: assertion" message when it should start to receive the video stream from the other host. So there is likely a relation between the message and displaying the video.

a progress report from last week - with locally built FF from trunk I can passively use Google Meet (can receive both audio and video), no change with Bluejeans. I'll keep you updated when there is something new.

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let's keep it open to track progress until things work out of the box