After installation of the security update that upgrades Firefox from version 78 to 91 on a openSUSE Leap 15.3 laptop with Intel graphics (Haswell-ULT / Core i5-4200U) and Xfce4 desktop, Firefox did no longer update its UI properly. Even for a newly created user. I can confirm that disabling hardware acceleration in Firefox settings solves this issue. I can also confirm that the workaround of Lukas in Comment #8 is working.