Reported in http://forums.mozillazine.org/viewtopic.php?p=14937575#p14937575 The NSFW web site(https://stripchat.com/) seems to be affected this bug.
Bug 1787127 Comment 9 Edit History
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Reported in http://forums.mozillazine.org/viewtopic.php?p=14937575#p14937575 You should turn on Windows10 OS dark mode with Firefox's default system theme. The NSFW web site(https://stripchat.com/) seems to be affected this bug.