Tl;Dr:
In about:config, I changed these preferences:
- widget.non-native-theme.gtk.scrollbar.round-thumb: false - This makes the scrollbar not have rounded edges
- widget.non-native-theme.gtk.scrollbar.thumb-size: 1 - This makes the scrollbar ‘chonkier’ within the scrollbar region
- widget.non-native-theme.scrollbar.size.override: 20 - This increases the scrollbar region size. Larger number = wider scrollbar
- Make sure widget.gtk.overlay-scrollbars.enabled is set to false - This should have been set to false when you enabled “Always show scrollbars”
On Windows, Firefox follows the system setting (System Settings > Accessibility > Visual Effects > Always show scrollbars).
I use a Wacom tablet in place of a mouse (for work, and because I get serious arm pain after about ~30mn of using a mouse) and while it has a "touch ring", it requires me to let go of the stylus and it's not very sensitive at all, making for a poor replacement of a mouse wheel. Some time ago I got myself a macropad with big rotary knobs and they do the job, but for years I relied on scrollbars.