I'm not sure why High Sierra is broken, but there are issues filed. See https://bugzilla.mozilla.org/show_bug.cgi?id=1379811 for the meta bug tracking all the ones we know about.

I'm guessing this will be fixed in a few days as people upgrade internally. So far, other than looking ugly, it doesn't seem to be causing any other problems.

I haven't encountered this, but it'd be great if you could report it: https://bugzilla.mozilla.org/

Who knows, the fix might make it in time for the actual release.

Switching the theme from the Default to Dark/Light does the trick for me, but yeah, definitely a minor nuisance.

I'm on High Sierra and I got the same issue!

Oh haha, oops - I guess I just ignore banner-like images. They don't talk about it in this link, I just noticed it.