We have a page in our application in which we allow for uploads of an excel file and then allow for a download of two generated excel files. In order to make the file upload work without two postbacks we added the enctype code (Me.Page.Form.Enctype = "multipart/form-data"). This fixed the two postbacks and everything seemed to be working in IE but then we found that in Mozilla one of our download buttons was returning a scrambled excel file. The following is part of the code that assigns the type and file extension to the generated excel file.
Private Const ExcelContentType As String = "application/vnd.ms-excel"
Private Const ExcelFileExtension As String = "xls"
Keep in mind that it completely works in IE, and that one of the buttons is working in Firefox but not the other.