Cannot Scroll Betwen Pages in Firefox (iMac w/ Magic Mouse)

Starting yesterday, I have not been able to scroll (back and forward) between pages with my mouse. I checked the settings, replaced the batteries, re-installed Firefox, restarted my Mac and still nothing. I thought it was just my mouse acting up, but then I realized it worked fine in Chrome and Safari, and that Firefox was the only culprit.

Anybody know what I can do to get 1-finger scrolling back in Firefox?!