The scroll wheel has no impact on popup windows in Cocoafox. Scroll events that occur over a popup window don't cause anything to scroll. This is most likely because popup windows are inactive. See also bug 339447. Testcase: Open an autocomplete dropdown popup that is scrollable, position the pointer over it, and manipulate your scrollwheel or whiz-bang two-finger touchpad, Expected behavior: it should scroll. Observed behavior: it doesn't. This is NOT Carbonfox bug 339705.

So it turns out this is due to Cocoa being lame. You only get scrollWheel: events for windows which are key. Making out popup window key is a non solution as it causes the titlebar of the window to change. It seems like installing a carbon event handler for kMouseWheelMoved would work if we installed it against the EventDispatcher target. We might be able to get it to work with just the Window target, but I don't know if these same things happen or not -- I haven't tested it yet. Advice welcome.

Fixed by patch for bug 387164, which just landed on trunk.