We are aware of a long-standing bug with Mozilla Firefox causing issues for users when they share their screens during their Call.Group web conferences.
We first reported these Firefox issues to Mozilla back in November 2020, and have since checked back with them periodically.
Unfortunately, as of February 2021 the issue remains unresolved.
We really regret the inconvenience caused to our users, and hope that Mozilla will be able to fix the bug soon.
In the meantime we strongly encourage users to switch to a different browser for their Call.Group conferences. In particular, we would recommend Google Chrome or Edge.