There is a known issue with Firefox and Mac OS X Lion with some scrollable regions. Mac OS X Lion has implemented a new scroll-on-demand feature that Firefox is passing-down to scrollable regions in its browser. This is on the official Firefox bug list. In the mean-time, there are two workarounds.
- Use your arrow keys: Click in the scrollable region and then use the arrow keys on your keyboard to scroll up/down and left/right.
- Change your Mac OS X Lion System Settings: Go to your System Preferences > General > Show scrollbars: and set to Always.