Webcam flicker in zoom with Qubes Librem 13

I have a Librem 13. I’m running Qubes. In a Debian 10 VM, I’m running zoom on firefox

My video flickers constantly, visible to me and to others.
When I view other people, I don’t see a flicker.
When I run cheese, I don’t see a flicker.

Does anyone have a solution to this problem?