If Firefox does not even try to load the file, the server might have sent it with a non-standard header. Two scenarios: (1) Normally, a server will send the content-type "application/pdf" ahead of the file. If the server sends a different content-type, then Firefox will not handle it as a PDF. (2) Normally, a server does not specify whether content can be viewed in the browser or must be saved. If the server send a header indicating that the content-disposition is "attachment" then Firefox will always display the download dialog. (There is an add-on you can use to work around this if you like: [https://addons.mozilla.org/firefox/addon/inlinedisposition/])