This problem has existed with both Firefox 13 and 14. Two finger scrolling works briefly, then stalls. Clicking above or below the scrollbar button doesn't work, nor does using the scroll arrows after the stall occurs--only grabbing the scrollbar button and dragging it works. After doing this, two finger scrolling works again briefly before the problem recurs.
This on my 17" Macbook Pro running OS 10.6.8.