Test Failure System.ArgumentException

Ran into this message on an automated test I ran today using FireFox.  Looks like it failed at the start of execution.  Any insights as to the cause?  The other odd thing is that while I was watching the screen during execution, opening and closing browser windows during navigation test steps, one of the FireFox windows appeared completely blank/white with the text "Preparing the browser for automation".  From this point on, each subsequent browser window opened in FireFox looked the same and none of them closed -- they simply stacked up one after another.  ??? 

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'3/16/2012 4:36:27 PM' - Using .Net Runtime version: '4.0.30319.239' for tests execution.
'3/16/2012 4:36:27 PM' - Starting execution....
'3/16/2012 4:36:49 PM' - Failure detected during execution. Details:
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'3/16/2012 4:36:49 PM' - System.ArgumentException: Hwnd of zero is not valid.
   at System.Windows.Interop.HwndSource.CriticalFromHwnd(IntPtr hwnd)
   at ArtOfTest.Common.Win32.Window.GetRectangle()
   at ArtOfTest.Common.Win32.Window.GetBitmap(Size windowSize)
   at ArtOfTest.Common.Win32.Window.GetBitmap()
   at ArtOfTest.WebAii.Design.AutomationHostState.FromBrowser(Browser browser, BrowserStateCaptureMode mode)
   at ArtOfTest.WebAii.Design.Execution.ExecutionUtils.GetHostState(IAutomationHost host, BrowserStateCaptureMode captureMode)
   at ArtOfTest.WebAii.Design.Execution.ExecutionEngine.CaptureCurrentState(AutomationStepResult currentStepResult, Exception ex)
   at ArtOfTest.WebAii.Design.Execution.ExecutionEngine.ExecuteStep(Int32 order)
   at ArtOfTest.WebAii.Design.Execution.ExecutionEngine.InternalExecuteSteps(AutomationStepList stepList, TestResult testResult, Object codeBehindInstance, Boolean& stopExecution)
   at ArtOfTest.WebAii.Design.Execution.ExecutionEngine.InternalExecuteTestIteration(Object codeBehindInstance)
   at ArtOfTest.WebAii.Design.Execution.ExecutionEngine.InternalExecuteTest(Test test, TestResult initializationResult)
   at ArtOfTest.WebAii.Design.Execution.TestExecuteProxy.ExecuteTest(ExecuteTestCommand command)
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'3/16/2012 4:36:51 PM' - Test completed!