That setting is controlled by Windows, not Firefox. It can be changed different ways. Probably the easiest way is to right click on a PDF file that you already have saved on your computer and select '''Properties'''. Under the '''General''' tab, there's an '''Opens with''' option that shows what program your computer is using to open PDF files. If you press the '''Change''' button and set it to your preferred PDF application, that should change only the PDF setting. Alternatively, on Windows 7, there is the option to change your default programs. This is usually listed in the right side of the start menu. If it's not, you should be able to find it by searching in the start menu. From that window you can select '''Associate a file type or protocol with a program''', find PDF and change the program. See the [https://support.microsoft.com/en-us/windows/change-which-programs-windows-7-uses-by-default-62fd162f-8c82-0436-806f-c60d69dcf495 Change which programs Windows 7 uses by default] page on the Microsoft website for more information. Hope this helps.