....wondering how many other websites are affected by such a major bug in a core CSS function.
That's what gives me faith that the Firefox folks will fix this one fairly soon. It's an obvious blunder.
(Off-topic, but I used to default to Firefox. It seemed to be smoother than Chrome, with better image and font rendering. Subtle differences, but still... But now I'm a convert to Brave. It's been described as "Chrome without the spyware." It uses the same engine, but it doesn't "phone home" to Google. Even better, ad-blocking isn't done with the overhead of an extension. It's baked into the browser, so even though it's very effective at blocking ads, it's also faster about doing it.)
