Stop Firefox From Opening PDFs in Two Apps

Firefox can show a PDF in its built-in viewer while a second PDF app opens the same file. The fix is to pick one handler in Firefox's settings, or to turn off the built-in viewer when you want PDFs to open externally only.

Start with the PDF entry under Applications in Firefox Settings. If both windows still appear, set pdfjs.disabled to true in about:config and restart the browser.

In this guide, I have covered:

Firefox opens your PDF twice? Choose one app

You click a PDF link and two windows appear: Firefox’s built-in viewer and your PDF application. Firefox is passing the file to both, and the fix is to make one of them the only handler.

Start in Firefox’s application settings. Pick your preferred PDF app there, test a file, and watch whether a second window still shows up.

1. Click the menu and select Settings.
2. Open General, scroll to Files and Links, and select Applications.
3. Find PDF Portable Document Format in the content-type list.
4. Select the application you want to use from the Open with menu.
5. Close the Settings page and test the PDF again.

You’ll know it worked when the PDF opens in the app you selected and nothing else appears. Close any PDF windows that were already open before you test, because changing the setting does not close them.

RELATED: Fix Firefox Not Opening Web Pages from Bookmarks

Turn off Firefox’s built-in PDF viewer

Sometimes the Applications setting alone isn’t enough. Firefox keeps rendering the PDF itself while the external app opens the same file, which usually means the internal viewer is still active.

The switch lives in about:config, and it applies to every PDF Firefox handles. Plan on using your external app from here on, not just for the file in front of you.

1. Open a new Firefox tab and enter about:config in the address bar.
2. If Firefox displays a warning, select Accept the Risk and Continue.
3. Search for pdfjs.disabled.
4. Set pdfjs.disabled to true.
5. Restart Firefox and open the PDF again.

The PDF should now open only in the external application. Firefox may save the file temporarily or show a download confirmation before handing it over, depending on the app you picked.

Setting pdfjs.disabled to true affects every PDF Firefox handles. Set it back to false when you want the built-in viewer again.

Open every PDF inside Firefox instead

Maybe you want the opposite: no external app at all. That’s the right setup if you read PDFs in a tab and don’t want Adobe, Preview, or Edge opening beside it.

Here you keep Firefox’s reader active and point the file type back at Firefox.

1. Open the menu and select Settings.
2. Open General, scroll to Files and Links, and select Applications.
3. Find PDF Portable Document Format.
4. Set Open with to Firefox.
5. Open about:config, search for pdfjs.disabled, and confirm that it is set to false.
6. Restart Firefox and test the PDF.

Firefox’s viewer should now be the only window that opens. Mozilla also publishes official support guidance for PDF application handling if you want to check your setup against it.

What to check if the problem continues

Start with a clean test. Close every PDF window, open a fresh tab, and click the link again. Firefox can keep the earlier application choice for a tab that was already open, so an old tab makes a working setup look broken.

If the duplicate window comes back, repeat the about:config check and make sure pdfjs.disabled matches the outcome you want. Use true for external applications only and false for Firefox’s built-in viewer only.

When the double window happens only with a PDF from one particular site, look at that site’s permissions before you change anything global. The Firefox site information guide shows where those controls live.

ALERT: Do not keep both options enabled. Choose either Firefox’s built-in viewer or an external PDF application as the single handler for normal PDF links.