Investigate to large values for Neodymium.getViewPortSize() for Firefox

[georgkunze]

When calling the method Neodymium.getViewPortSize() the values returned for Firefox actually exceed the actual displayed viewport.
This matters so far as some of the image processing methods we use for Screenshots are reliant on correct values for the size of the viewport.
The reason the underlying javascript commands return incorrect values is unknown an need to be investigated.

Note: for Chrome and Edge the returned values appear to be correct.