To enable or disable the built-in PDF viewer in Mozilla Firefox, follow these steps:
- Open Firefox and type "about:config" in the address bar. Press Enter.
- You will see a warning message. Click on "Accept the Risk and Continue" to proceed.
- In the search bar at the top, type "pdfjs.disabled".
- You will see a preference named "pdfjs.disabled" listed below. Double-click on it to change its value.
- To disable the built-in PDF viewer, set the value to "true".
- To enable the built-in PDF viewer, set the value to "false".
- Close the about:config tab.
After making this change, Firefox will either use its built-in PDF viewer or download the PDF files, depending on your preference.