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In KDE 5.40 on openSUSE Tumbleweed, I have configured scrollbars to look and work like this:
However in Firefox, scrollbars are different. Note the lack of buttons at the end of each bar and being too slim:
The solutions from How do I change the width of scrollbars in Gnome? unfortunately only work on applications from ca. 2009.
I think firefox ignores the theme / window manager settings and uses whatever scrollbar it prefers. Only vaguely related, but this Q on AskUbuntu might be worth trying, basically
– Xen2050 – 2017-11-28T11:41:43.757echo -e "[Settings]\ngtk-primary-button-warps-slider = false" >> ~/.config/gtk-3.0/settings.ini
(if the file already exists take a look at it first) + restart Firefox, and it's easy to undo if it doesn't work