Firefox does not recognize .xls download (PHP)

I have the following headers in a php view

header('Content-Type: application/xls');
header('Content-Disposition: attachment;filename="' .$user[0]. " " .$user[1]. '".xls'); 

with the chrome browser the file is downloaded working correctly but with firefox when downloading, download without format, how could you solve that detail?

I would appreciate the interest.