Reading other users' reviews, there's quite a lot of MOANING that Dev (Wesley) hasn't coded in enough options.
Note: I'm using LibreWolf but I'm sure it'll be identical in FF.
In about:config, type in 'scrollbar' to get you to the right area, then try the following:

'widget.non-native-theme.scrollbar.style' - this affects the scrollbar style, it goes from 0-4. '3' is thicker and rounded, '4' is thickest and squared off.

'widget.non-native-theme.scrollbar.size.override' - affects the thickness in 3 and 4 above. Anywhere between 10-20 gives you nice thick bars.

I had a play around with all the 'scrollbar' settings and those seem to be the ones for a decent sized bar. How about having a play around yourselves in future, before giving Dev a hard time for creating a brill add-on xx