RegisterClientScript for window.open Not Working in Firefox

I have the following line used to download a file after the user clicks an export button.  I need the code to work in IE8 and Firefox 20+.  The following works great in IE8 but not Firefox.  Ideas?

ClientScript.RegisterClientScriptBlock(GetType(String), "download", "window.open('" & MyWriter.PostedFile & "','download');", True)

The PostedFile value is in the format file://networkdrive/filename.xlsx.