Firefox wayland UI rendering freeze on video playback, CLUTTER_PAINT=continuous-redraw works around the problem
See https://bugzilla.mozilla.org/show_bug.cgi?id=1653850#c23 I think this is a clutter bug, because setting CLUTTER_PAINT=continuous-redraw makes the problem go away. See https://bugzilla.mozilla.org/show_bug.cgi?id=1653850#c22 for steps to reproduce (hover mouse over video playback on the linked site in the comment) on Fedora 32.
Is there any more debug info I could collect when the issue is happening to track down where the bug is? WAYLAND_DEBUG=1 on firefox-wayland didn't seem to yield anything particularly different when the bug occured or not.