Indeed according to the upstream bug report, and to the associated KDE bug report (https://bugs.kde.org/show_bug.cgi?id=428499), this should be fixed in kwin 5.21.0, which is in Ubuntu 21.04.
** Bug watch added: KDE Bug Tracking System #428499 https://bugs.kde.org/show_bug.cgi?id=428499 ** Summary changed: - Firefox unusable on Kubuntu 21.04 Wayland session + Firefox is repainting only on re-focusing in Kubuntu 21.04 Wayland session -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1922438 Title: Firefox is repainting only on re-focusing in Kubuntu 21.04 Wayland session To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1922438/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs