The recent Firefox Developer Edition update (91.0b6) seems to have broken CSS rendering or something. I’ve seen backgrounds and inset shadows missing where they should be, extra backgrounds where they shouldn’t be, link colors not being applied and showing as default, other colors being off. I’ve been noticing more bugs in the developer tools and elsewhere in the browser over the past couple years, but this one really makes it unusable. Hopefully they can get it fixed soon, because I normally spend a lot of time working on sites with that browser.