For what it is worth, another user had this problem. He inadvertently right clicked on the toolbar (where "File, Edit, View..." is displayed and clicked on "Customize...". When the customization window opened the right scrollbar widened. I did the same with the same results.
It is possible that the window width wasn't correct and that opening the customize window corrected that.
I've spent a lot of time trying to find an understandable way to widen the tiny, narrow scrollbar that's giving me fits. Why is it so very narrow? it's extremely difficult to use. I've read something about a weird CSS code but can't even get an understanding of that. This is all very complicated, even for an advanced user like me. Thanks for any help you all can give me. ec
Go to ( previously Display properties-Windows Color-Advanced) Personalization-Window Color-(tiny bottom link) Advanced Appearance settings. Click on the scrollbar or select scrollbar from dropdown menu. Change size, APPLY!, reboot Firefox. Readjust as needed.