Wrong file extension when saving a PDF in Firefox

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ASP.NET

Javascript

On a button click I'm opening a PDF by this code-

function LoadPreview()
    {
        var theFeats =         "height=800,width=650,location=no,menubar=no,resizable=yes,scrollbars=yes,status=no,toolbar=no";
          window.open("Preview.aspx?IsUser=1", "Preview",theFeats);
            return false;
    }

and rest code is written in Preview.aspx.It's working fine fore Chrome and IE.
In Firefox pdf document header is coming "Preview.aspx"
and I save that doc then name is coming "Preview.aspx".
and document is saved with .aspx extension.

How an I solve this Problem.