How To Enable Integrated PDF Reader In Firefox 18

How to enable the integrated PDF Reader in Firefox 18 which was released this week. One feature which still hasn’t made it officially ‘live’ is the PDF Viewer – this lets you view PDF files within Firefox so there is no need to open a separate program like Adobe Reader or save the PDF first.

Although this feature is included, it is currently disabled and therefore not officially supported – expect it to be enabled by default in a future version. For those who can’t wait and want to give it a try, you can enable it right now.

How To Enable Firefox Built In PDF Viewer

  • Open Firefox and type about:config in the address bar and press Enter. Click the “I’ll be careful I Promise” warning button to reveal a long list of preferences used in Firefox.
  • Search for (see tip below) or scroll down to the preference named “pdfjs.disabled” – the default Value is ‘true’ i.e. do not use the built in Viewer.
  • Double click it to change the Value to ‘false’.
  • Close the about:config tab – the new integrated PDF Viewer is now enabled. See the next section for how to use it.

Tip: You can scroll down the list of preferences but the quickest way to find it is to type part of the preference name e.g. pdfjs into the Search box at the top of the config page.

Configuring The Integrated PDF Viewer – You also need to change Options to make PDFs open within Firefox:

  • Click the Firefox button (or Tools on menubar) then select ‘Options’ and click on the ‘Applications’ tab.
  • Scroll down the list of Content Types to ‘Portable Document Format (PDF)’ and, in the Action column, select ‘Preview in Firefox’ from the drop down list (shown below) then click OK to close the Options window:


Firefox is now configured to use the integrated PDF Viewer by default. To test it, browse to a website containing a PDF file and, when you select the PDF, it should now open within a Firefox tab.

If you have any problems (bear in mind this is an unofficial feature) just revert back to not using it – follow both processes above but this time double click the preference to turn it back to ‘true’ and change the Action column back to your desired choice e.g. Save File or to use whichever PDF Reader you have installed.

Features of Firefox PDF Viewer – The viewer includes the most basic features of any PDF reader as shown in the menu bar below:

Firefox PDF Viewer – top menu bar

Starting from the left hand side, the features include the following:

  • Toggle the Slider – display a left sidebar with a choice of viewing Page Thumbnails or Document Outline (e.g. Chapter/section headings).
  • Previous/Next page buttons and selection for jumping straight to a specified page.
  • Zoom in/out buttons and drop down menu of zoom options e.g. automatic, page width or zoom percentage.
  • Switch to Presentation mode – displays the PDF in full screen and hides all other content.
  • Print – brings up the standard Windows print dialog box.
  • Download – save the PDF file to computer, brings up the standard Windows dialog box to confirm/edit the name of the file and choose the save location.
  • Current View – changes the URL in the address bar to the current position within the document so you can copy it or open the URL in a new window/tab.

The actual display of PDF files worked well in my testing, even for graphically intense and complex documents, but scrolling is not as smooth as in external programs.


The ‘unofficial’ built in Viewer isn’t a direct replacement for an external program but it is a neat way to more quickly view, print or save PDF files – without having to wait for an external Reader to launch. It may lack the advanced features of standalone programs like Adobe Reader but it works well enough, at least for simple files.

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