Annoying Firefox scrolling problem

(I know this isn't a Firefox support site, but I thought I'd ask...)

I'm getting an annoying problem in Firefox (Bon Echo 2.0.0.18 on BeOS R5): when I drag the scrollbar to scroll, then release the mouse button, occasionally it acts as if the mouse button is still down and scrolls if I move the pointer up or down. It only seems to happen in Firefox and I've tried multiple computers and multiple "mouses". I've checked Firefox options to see if this is a "feature", but I don't see anything.

Does anybody else have this problem? Is there a solution?