(Press ⌘, if you are on Mac)In the search bar under the settings menu, enter PDFScroll down and click on Portable Document Format (PDF) under Applications. How to Open PDF Files in Your Favorite •. After I download any PDF, Firefox immediately opens it in a new tab. Firefox opens any PDF that I download. In case that you've turned off the PDF or it's not enabled, you can always go switch it on by following these steps: Go to Menu > Download s or press CTRL+J to open your downloads. ,  · How to