Superb info in the OP - sincere thanks @Radax05 !!
My FF did not have the line
widget.non-native-theme.scrollbar.size.override
But I did the others & it does make the scrollbars WAY more user friendly.
My 27" display is also set to 1360 x 768 as with Jymm and that is fine for most everything I use - but FF was an annoying exception, now fixed.
Aside of that I must add that my only usage anymore for FF itself is for a single video source which was difficult when scrolling info there until now.
My main, 'daily driver' browser is Basilisk, 2nd to that is Librewolf, I also use an older version of the non-Linux Mypal every day - ungoogled Chromium when I am forced to - so FF comes in around 5th place for me.
Once it became chrome-ized I really lost interest in using it.
Basilisk uses the original add-ons and with the addition of the Classic Addons Archive as well as Classic Theme restorer it brought browser sanity back for me - rather than the lobotomized & mostly un-customizable baloney that FF itself has become.