Hi chispa,

The dialog filter function is based on silverlight and flash plug-in. On your firefox, it will loads the html5 as the default upload mode. The control will check the file extension again at server side, so i think it will not causes the problem. 

you can set property  UploadTypePriority to Silverlight,Flash,HTML5, then the control will loads the silverlight and flash first.

     <CuteWebUI:Uploader runat="server" ID="Uploader1" UploadTypePriority="Silverlight,Flash,HTML5" >
        </CuteWebUI:Uploader>
 

Regards,

Ken