I am using Firefox 56 from the Mozilla OBS repo in GNOME on Leap
42.3. I have also tried this in the version of Firefox 56 that is
currently shipped with Tumbleweed (on KDE) and the issue is the same.

I noticed that the new Firefox now uses a gray window background by
default.
This interestingly breaks quite a few websites, because at some point
(around 2000), all relevant browsers switched from default of gray to a
default of white for their window background. And then web authors
became lazy and stopped providing a background color when all they
wanted was white...

For a website that looks off because of this, try e.g. the login screen on
https://login.microfocus.com (obviously not ideal if you're already logged in
to Bugzilla.)

(But there is a myriad of other random sites that suddenly look wrong, say
https://emojipedia.org/ .)