1. go to about:config
2. find widget.non-native-theme.scrollbar.size.override
3. Change to some pixel value (I have mine set at 26).
voila!
I do not understand why the default is so thin. Really bucks the trend of every UI element being touch-screen large nowadays!
For scroll/touch users, the scrollbar has become an indicator only, and not so much a tool for scrolling, to show where on the document the window is.
If it's literally anywhere else on the screen, then no, Fitts's Law applies, and it's a bad target. It has height but no width.
This is especially true for those with motor or visual handicaps, or both.
It's damned near unusable even with very modest levels of distraction or other factors, e.g., using a device as a passenger on a moving vehicle, as with a boat, train, or airplane.
It is. Visually there's space between the scroll handle and the window edge, but if you actually point the cursor in that space it'll highlight the handle.
I guess thinking about it this is a very reasonable compromise, but it feels weird to start scrolling up and down with a cursor that shows a left-right arrow and that is technically pointing outside of the scroll bar.
It ... doesn't work for any Linux desktop, most especially those using Focus Follows Mouse (certainly for strict, often for lazy) policy.
Though that's useful to know.
Though I'm using click-to-focus, not focus-follows-mouse, so YMMV.
If I have a window positioned to the right edge of the screen ... the scrollbar is draggable only when my mouse pointer is positioned directly over the scroll grabber. Not when pushed to the edge of the screen.
This seems to apply generally for windows best I can tell.
TL;DR: The behaviour you describe is not at all what I see.
My preferred informational / programming layout is browser on left, terminal on right.
And in any multi-window layout, any window not aligned with the right-hand display border remains difficult to target.