This problem posted 2 years ago was not answered and I found that I am having the same problem.  I found that when I enter a Export Options - Custom File Name Firefox is unable to open a file originating from a table in Excel.  I have customers that insist on using Firefox instead of using IE 11 or Chrome.  Our Chrome is locked down and allows only the minimum capability. 

Through trial and error I found when the Export Options - Custom File Name blank, Firefox is able to open the csv file using the MS Excel.  However, when I populate the Export Options - Custom File Name and then try to export, Firefox cannot open with Excel, but instead requests the user browse for an application to open the file.