According to your User Agent you posted this question using Internet Explorer 11 on Windows 8.1. Are you having a problem with Firefox? What Firefox version? You said you are having a problem opening PDF files on your system. Can you explain that better? Mozilla Support can help you with opening PDF files in Firefox, which uses a built-in PDF reader. Maybe one of these articles will help? * [[View PDF files in Firefox without downloading them]] * [[How to disable the built-in PDF viewer and use another viewer]]