Ah, I figured it out. This was on a Thinkpad, by the way - maybe the Thinkpad specific Synaptics drivers have an outdated .dat file or something. Anyways, all I had to do was open the tp4table.dat file in my Synaptics folder, and add this line:
*,*,firefox.exe,*,*,*,WheelVkey,0,9
Then I killed my SynTPEnh process and restarted it, and horizontal scrolling worked perfectly.