I can not open PDF files within Firefox, nor can I open them in an external program like Adobe Acrobat Reader. They automatically save to my Downloads folder instead. I have already changed the settings for "Portable Document Format (PDF)" under Options > General > Applications to be "Preview in Firefox" and I have also tried changing it to "Use Adobe Acrobat Reader DC". Neither seem to be working. It still downloads the file instead of opening it. I have also verified that there are no extensions that may be affecting the way the setting behaves, and there is nothing.