mousewheel event is not triggering in firefox browser

This is 2017 and the right answer is now:

$(window).on('wheel', function(event){

    // deltaY obviously records vertical scroll, deltaX and deltaZ exist too
    if(event.originalEvent.deltaY < 0){
        // wheeled up
    }
    else {
        // wheeled down
    }
});

Works with current Firefox 51, Chrome 56, IE9+

Note: The value of the deltas will depend on the browser and the user settings.