I have built a website for a client using Concrete5 as the CMS.
There is a download link on this page which links (via Concrete5's file manager code) to an xlsx file.
If this link is clicked on in Google Chrome or Internet Explorer, the file downloads as expected and everything is A-OK.
If this link is clicked on in Mozilla Firefox, however, the user is instead presented with the raw source of the .xlsx file, and no option to download it.
The headers and content the server is serving appear identical, so I can't understand why the two browsers would behave differently.