Then I would question whether the file was shared correctly by the uploader. I shared a file from my Dropbox, setting it so anyone with the link could view, and was able to download it from desktop view on mobile Firefox (in incognito mode)
There is an option to require a password to view the file, is there any possibility that that was set in your case?
Only other thing I can think of is the specific mobile device/browser combo isn't properly requesting the desktop site. Or there is somehow an entirely separate sharing option that is functioning differently