Very worrying trend is thin scrollbars on websites, or in some rare cases - scrollbar just hidden. Fortunately Firefox has a setting in about:config: layout.css.scrollbar-width-thin.disabled=true .

There's also

  widget.gtk.overlay-scrollbars.enabled: false - always show normal scrollbar, don't just expand on hover
  widget.non-native-theme.scrollbar.style: (it seems that values 1-5 are allowed)
  widget.non-native-theme.scrollbar.size.override: width
Works in Thunderbird too.

yes, I absolutely hate the new trend of tiny slivers that are basically invisible

I just want normal OS-controlled scrollbars everywhere