Hello there,

Type about:preferences#applications<Enter> in the address bar. What is the information for those type files?

Did you want to show the file in Firefox, or another viewer?

For more details please view this topic: https://support.mozilla.org/en-US/questions/1045938

Hope this helps

When you enter PDF in the search box above the list, Firefox should show just Portable Document Format. If that's missing, the file that stores the application handling preferences might have become corrupted. Since the file is difficult to edit, we generally suggest renaming it and letting Firefox start a new one. Here's how:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" button > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter/Return

In the first table on the page, click the "Show in Finder" button. This should launch a new window listing various files and folders.

Leaving that window open, switch back to Firefox and Exit, either:

• "3-bar" menu button > "power" button
• (menu bar) File > Exit or on Mac it might be Firefox > Quit

Pause while Firefox finishes its cleanup, then rename mimeTypes.rdf to something like mimeTypes.old (in case you need to recover it later).

Start Firefox back up again, and return to the Application preferences to select your preferred viewer. Is Portable Document Format listed?

Note: if you previously customized the setting for email links, you'll need to fix that one, too. See: Change the program used to open email links.

I enter about:preferences#applications <Enter> in the address bar, I searched for PDF, or Portable Document, neither of them returned anything. The only one I have is Adobe Acrobat FDF document. OK, then, I see "Preview.app Document", changed from "Always ask" to "Use Adobe Acrobat Reader", that fixed the problem.

Thanks.