You may have issues viewing .pdf files in Firefox. This is most commonly caused by two issues.
Issue #1 – You don't have Adobe Reader installed
Solution – Install Adobe reader from http://get.adobe.com/reader/. Be sure not to install any optional software they offer. Most of the time you have to uncheck a box to reject optional software. Note: You will need administrator access to install Adobe Reader if this is an ASU device.
Issue #2 – Default applications in Firefox are set wrong.
Solution – When in Firefox click the three lines hamburger menu at the top right of the browser. Under the General tab of the options menu, scroll down until you see "Applications" and "Portable Document Format (PDF)". This option needs to be set to either "Open in Firefox", or "Use Adobe Acrobat".