[Firefox] Mouse scroll wheel doesn't move volume slider

[maciex]

The problem is caused by the 'mousewheel' event being deprecated.

    Non-standard
    This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.```

I changed the volume slider code to use 'wheel' event.