You can try passing the -url parameter along with that code of yours when opening a new Firefox process.
Dim NewProcess As Process = Process.Start("firefox.exe", "-url ""javascript:window.open('" & url1 & "','MyWindow','config=toolbar=no,menubar=no,scrollbars=no,resizable=no,location=no,directories=no,atus=no');""")
EDIT:
Alternatively (since this does not seem to work in all cases) you could write a temporary HTML file which when opened creates a new window per your specifications.
Public Const BaseWindHTML As String = "<script>window.open('<replaceurl>','MyWindow','config=toolbar=no,menubar=no,scrollbars=no,resizable=no,location=no,directories=no,atus=no'); window.close();</script>"
Public Sub OpenNewWindow(ByVal URL As String)
Dim HTMLPath As String = IO.Path.Combine(System.AppDomain.CurrentDomain.BaseDirectory(), "newwindow.html")
Using Writer As New IO.StreamWriter(HTMLPath, False)
Writer.Write(BaseWindHTML.Replace("<replaceurl>", URL))
End Using
Dim fProcess As Process = Process.Start("firefox.exe", "-url """ & HTMLPath & """")
fProcess.WaitForInputIdle()
Threading.Thread.Sleep(1500)
fProcess.CloseMainWindow()
End Sub
Example use:
OpenNewWindow("http://www.google.com/")