Make Firefox Open PDFs in the Built-in Viewer

Firefox includes a built-in PDF reader, pdf.js, that opens PDF files in a browser tab instead of downloading them. When PDFs start downloading or open in another app, the PDF content type is usually set to the wrong action in Firefox's Applications panel.

You can fix that with one dropdown change. Set Portable Document Format (PDF) to Open in Firefox, and every normal PDF link opens in the built-in viewer on your computer.

In this guide, I have covered:

Why Firefox Stops Opening PDFs in the Built-in Viewer

You clicked a PDF link and expected the file to open in a tab. Instead, Firefox downloaded it or handed it to Adobe Acrobat. The browser usually is not broken. The PDF content type is set to the wrong action in the Applications panel.

Firefox ships with a built-in PDF reader called pdf.js. It opens PDF files in a browser tab without a separate app. The fix is one dropdown change in Settings, and it only takes a minute.

Make Firefox Open PDFs in the Built-in Viewer

You only need to change one dropdown in Firefox’s Applications panel. The path and wording are the same on Windows, macOS, and Linux.

1. Open Firefox on your computer.
2. Click the hamburger menu in the top right of the window.
3. Select Settings from the menu.
4. Choose General in the left sidebar if it is not already selected.
5. Scroll down to the Applications section.
6. Type PDF in the search box above the Applications list to filter the entries.
7. Locate Portable Document Format (PDF) in the filtered results.
8. Click the action dropdown to the right of the PDF entry.
9. Pick Open in Firefox from the dropdown.

The change applies right away. The next PDF link you click should open in a new tab using Firefox’s built-in viewer instead of downloading or launching another app.

TIP: Power-user shortcut: the next section shows a one-line address bar route that lands on the same Applications panel without scrolling through General.

Use the Address Bar to Jump Straight to the PDF Setting

If Firefox lists many content types, scrolling to the PDF entry gets old. A special address bar URL drops you on the Applications panel in one step.

1. Click the address bar at the top of Firefox.
2. Type about:preferences#applications and press Enter.
3. Find Portable Document Format (PDF) in the Applications list.
4. Click the dropdown next to it and select Open in Firefox.

You land on the same Applications section as the menu path. The PDF entry and dropdown options are identical.

Verify the Built-in PDF Viewer Is Working

You changed the setting. Now confirm the viewer actually loads a PDF. A quick test catches cases where the change did not stick or another preference overrides it.

1. Open a new tab in Firefox.
2. Visit Mozilla's PDF.js test document at https://mozilla.github.io/pdf.js/web/compressed.tracemonkey-pldi-09.pdf.
3. Confirm the PDF renders inside the tab with a toolbar that includes zoom, page navigation, and a download icon.
4. Use the toolbar to scroll through pages, zoom in, and try Print to confirm the viewer is fully active.

If the PDF opens in the tab and the toolbar appears, the built-in viewer is working. For more on what the built-in viewer can do, see Mozilla’s official guide on viewing and editing PDF files in Firefox.

If Firefox still downloads the file or opens a different app, return to the Applications panel and recheck the dropdown selection. Then use the troubleshooting steps below.

Other PDF Actions Available in Firefox

Firefox can handle PDFs in four ways. Knowing what each action does helps you pick the right one if you want to switch back later.

• Open in Firefox: opens the PDF in a new tab using the built-in viewer. This is the option you set in the steps above.
• Save File: always downloads the PDF to your default download folder without opening it.
• Always Ask: Firefox prompts you every time you click a PDF, asking what to do.
• Use <app name>: opens the PDF in a system application such as Adobe Acrobat, Preview, or another installed reader.

You can switch between these at any time. The choice is not permanent and is easy to change back through the same Applications panel.

If your dropdown only shows Preview in Firefox, you are on an older Firefox version (pre-98). The label was renamed to Open in Firefox in early 2022, but both options use the same built-in viewer.

What to Do If Firefox Still Downloads or Opens PDFs Elsewhere

Sometimes the dropdown reads Open in Firefox but PDFs still download or open in another app. A few common causes explain it.

Check for a Conflicting Extension

A PDF-related extension, such as a download manager or a third-party PDF tool, can override Firefox’s built-in handling. Disable extensions one at a time to find the culprit.

1. Click the hamburger menu and select Add-ons and themes.
2. In the left sidebar, click Extensions.
3. Use the blue toggle next to each extension to disable it. Start with ad blockers, tab managers, and any extension you installed in the past few weeks.
4. Restart Firefox from the hamburger menu .
5. Use Firefox normally for a few minutes, then check about:performance. If usage dropped sharply, re-enable extensions one by one to find the offender.

Reopen a PDF link after disabling extensions. You’ll know it worked when the same PDF opens in the tab. If it does, re-enable extensions one at a time to find the one interfering, then keep it disabled or remove it.

Clear Site Data for the Problem Site

If a specific website’s PDFs behave strangely, leftover site data can override the global setting for that domain. Clearing the data gives Firefox a clean slate.

1. Open the menu at the top right of Firefox and click Settings.
2. Select Privacy & Security from the left sidebar.
3. Scroll to the Cookies and Site Data section and click Manage Data….
4. Type the site's name into the Search websites field to find it faster.
5. Select the site, then click Remove Selected.
6. Click Save Changes and confirm the prompt.
7. Reopen the site in a new tab and pin it again.

Try opening the same PDF again after clearing the site’s data. You’ll know it worked when the same PDF loads in the viewer. For broader cleanup, you can also clear the cookies and cache for Firefox as a whole.

Refresh Firefox to Restore Default Behavior

If the setting refuses to stick or Firefox keeps reverting to Always Ask, a refresh restores most preferences to their defaults while keeping your bookmarks, passwords, and history. For a deeper reset, you can reset Mozilla Firefox browser settings on your computer.

1. Click the hamburger menu , choose Help, then select More Troubleshooting Information.
2. On the Troubleshooting Information page, find the Give Firefox a Tune Up section on the right.
3. Click Refresh Firefox and confirm by clicking Refresh Firefox in the dialog.
4. Wait for Firefox to restart. Your bookmarks and logins are preserved, but extensions and most site settings are reset.

After the refresh, repeat the main procedure above to set the PDF action back to Open in Firefox. You’ll know it worked when Firefox restarts and the Applications panel holds your new selection.

If you want a completely clean installation and are ready to start over, you can uninstall and delete stored files of Mozilla Firefox from a computer before reinstalling. This is a last resort for cases where the current profile keeps misbehaving.

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