Firefox 19 Released With New Integrated PDF Viewer

Firefox 19 has been released with a new built-in PDF Viewer and many other technical improvements. It is available now as an update (via Firefox Menu \ Help \ About Firefox \ Check For Updates) or via the full download at here.

Built-In PDF Viewer – this is the only new feature to make a public appearance (it was previously available by tweaking about:config). It lets you view PDF files in Firefox (in a normal tab) without the need for any third party PDF plugin i.e. no need for a plugin from programs like Adobe Reader.

It is now the default PDF Viewer but you can change that behavior as we shall see later.

When you select a PDF from a website, it will now open in a tab with the built-in Viewer as shown below:

PDF Viewer menu bar

The yellow warning message gives you the chance to open the PDF with your third party viewer (Adobe Reader or similar) if you prefer.

PDF Viewer Features – starting from the left hand side of the menu bar shown above, the features include:

  • 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. This 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.

So can you do away with a third party PDF viewer completely?

Well, you can now use Firefox to open not just online PDFs but also to open any PDF stored on your PC (just change the default PDF file association in Windows to Firefox or else right click on a PDF and choose Open With \ Firefox).

However, I don’t think most people should uninstall their third party viewer just yet – the display of small PDF files worked well in my testing but some large or complex PDFs were more sluggish and problematic – I ended up opening them in my third party viewer anyway.

How To Disable The PDF Viewer? If you really don’t want to use the new Viewer, do the following:

  • 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 ‘false’ i.e. use the built in Viewer.
  • Double click it to change the Value to ‘true’.
  • Close the about:config tab – the new integrated PDF Viewer is now disabled and PDF opening will revert back to your previous third party viewer preferences.

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

Other Changes In FF 19 – We have concentrated on the Viewer because it is the only new feature. However there are a variety of other changes and technical improvements including:

Startup performance improvements – a couple of changes to the order in which code is loaded should improve browser startup times.

Developers get a number of improvements including: Browser Debugger available for add-on and browser developers (experimental, set to true). Remote Web Console is available for connecting to Firefox on Android or Firefox OS (experimental, set devtools.debugger.remote-enabled to true).

There are also some HTML5 enhancements, many important fixes and a number of critical security fixes. For a full list of changes, see the official Release Notes here.

New Features Present But Still Disabled In FF 19 – These are new features introduced in the last few releases which haven’t yet been enabled.

Whilst they work fine for me it is possible they could have outstanding bugs but more adventurous users (or those wishing to get an early view of future features) can choose to enable them – they can easily be disabled again if they cause any issues:

1. Click to Play – this feature (configure all plugins to only load on click) is still not enabled by default. However, you can activate it via a change in about:config – see Activating Click To Play and Add a Button to switch Click To Play on or off.

2. In Content Preferences – this feature (Firefox Options open in a new tab instead of a separate window) is also still not enabled by default. However, you can activate it via a change in about:config – see Enable In Content Preferences.


The new built-in PDF Viewer is great for quick PDF viewing and is arguably more secure than third party viewers (because it lacks several extra features that add insecurity). It also doesn’t need to be kept up to date separately which is also good for security – no need to rely on third parties like Adobe for updates.

However, users may still find that large or complex PDFs display better in third party viewers so it may be best to keep both options open – use the new Viewer but choose ‘Open With Different Viewer’ in the event of problems.