EDIT: I have solved this problem on my end. It turns out that my mouse manufacturer had a 'helper' program that was screwing up the horizontal-tilt support. Quitting it solved the problem. To others having the same problem, I suggest checking your own mouse's helper utilities. - I am given to understand that previous versions of Firefox supported horizontal scrolling when used with a scrollwheel mouse that supported tilt messages. However, I've been unable to get it to work on Firefox 20.0.1. I've tried disabling smooth scrolling, hardware acceleration, restarting in safe mode, but it doesn't seem to make a difference. (The troubleshooting information below represents the usual configuration of my browser, without disabling any extensions.) On the other hand, other applications seem to handle mousewheel tilts just fine; I've tried it with applications as simple as Notepad on Windows and they scroll horizontally in response to mousewheel tilts. I'm stumped; did Mozilla remove tilt support in later versions of Firefox?