Some time ago the scrollbar became dark-themed for some reason:
Turns out it is some firefox feature that adapts the scrollbar to the page (https://www.ghacks.net/2019/11/11/firefox-72-dynamic-scrollbars-based-on-page-color/) based on the page background color. This forum colors are light but the background color is set to #585858, which is considered dark by firefox.
To fix that one can either disable the feature in about:config?filter=widget.disable-dark-scrollbar
or use userContent.css to override background to #767676 or lighter.