modalpopupextender opens in firefox when uploading a file using

I have an  <asp:FileUpload ...> in an update panel as well as an modalpopupextender.  In IE, I can select a file and then click the <asp:button ... upload file button, and everything works great.
 
In firefox, when I browse for a file and then hit the select button, the modalpopupextender displays and I do not know why.  The target and cancel controls sof the popup extender are not tied to the asp file upload tag, and I don't know why it desides to show once I select the file.
 
Any ideas on why this is hapening and how to prevent it would be great.
 
Thanks
...