Flying was right - solved my problem!
It turns out Flying had posted the solution to my problem. I am using a KVM switch. Unplugging my mouse and plugging it back in solved the problem.
I would have suspected that my KVM & mouse was the problem sooner, but I have not had any problems with scrolling in other applications or while using the other computers on the KVM switch. In fact, I also use Mozilla on a Windows-based PC connected to the same KVM switch and have not had a problem with it.