I sent a note to our internal lists today and already got a half dozen "I thought it was just my system..." responses, so for other people suffering in silence, and not recognizing this, I'll post it here too:
There's a bug in Xorg 6.9 (included in Nevada builds 28 & later, S10U2, and S10 patches) in which if you scroll the wheel too fast on your wheel mouse, you get left/right wheel events instead of up/down, which is particularly annoying in Firefox, which interprets left/right wheel as back/forward page history.
From recent conversation, it appears a lot more people have been suffering in silence with this bug than have reported it to us.
Change or add this line in the "InputDevice" section for your mouse in /etc/X11/xorg.conf:Option "ZAxisMapping" "4 5 4 5"(The default ZAxisMapping in Xorg 6.9 is "4 5 6 7".)
With the workaround, Xorg will still think it's getting left/right events, but will report them to applications as simply more up/down events. This will break people actually using left/right capable devices, like the internal-frkit enabled Ferrari 3400 scroll pad/button/thingy. The actual fix will allow both types of scroll events to work correctly, and has been put into our sources for Nevada build 40.