Issue with changing the resolution of video stream sent by Firefox

Scenario:
A Firefox joins a conference (with Chromes and maybe other Firefoxes).
Upload of this Firefox is above 2000 kps but resolution shown at the other participants is bad. It seems it is not increased, when the Firefox video is put into the front (i.e. from the side frame to the main frame).
When the other participants leave and rejoin the conference then the behaviour of this Firefox is fine, i.e. the resolution displayed at the others is adapted to whether this Firefox is in the main frame or only in a small side frame.
Is this issue known, how can it be avoided, where can I get additional information to identify the root cause?
The behavior is independent on the maschine where the Firefox is run. It happens with every Firefox that joins a conference whereas the video reolution of the Chromes shown at others (including Firefoxes) is always properly adapted.

My setup: a self hosted jitsi-meet (last updated from stable at April 19th) running on Ubuntu, Firefox release 75.0