Enable the Built-in PDF Reader in Firefox 15

Alan Buckingham
Sep 8, 2012
Updated • Dec 3, 2012

Mozilla recently rolled out Firefox 15, the latest version of the browser in their new rapid-release schedule.  With this new release there are quite a few new features, which have been outlined in the above linked post.  One interesting feature that bears a little more attention and some explanation is the new built-in PDF reader.

Building a PDF reader right into the browser is important for a couple of reasons - one is that it speeds up the execution of these files online, but the other is security.  Previously users have mostly plugged the Adobe PDF Reader into their browser because it's well known and it's also the first option that comes to mind for many.  However, the Adobe app is notorious for it's security problems.  With the new Mozilla feature we will, hopefully, get a more stable and secure way to view these files online.

Before we get started you will need to make sure you are running Firefox 15 (or higher if you subscribe to the beta).  To do this you can simply click the "Help" option on the menu bar and choose "About Firefox".  This will not only display the currently installed version, but also prompt you to update if you aren't running the newest build.

Now, in the address bar, type "about:config" and hit "Enter".  If this is the first time you have accessed these settings then you may be surprised by a pop-up message that will warn you about making any changes.  Don't panic, just click "I'll be careful, I promise!"

In the search bar type "pdfjs.disabled" and then right click this option.  In the context menu choose "Toggle" to change the setting from "True" to "False". You can alternatively double-click the entry.


You have now enabled the the PDF reader in Firefox.  Browse to a site that has a ".PDF" extension and try it out.  We have high hopes that this will solve the previously mentioned security woes that came with Adobe and perhaps make the browsing experience a bit faster as well.


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  1. Jim said on September 11, 2012 at 3:50 pm

    I’ve got an update on this. Yesterday I turned this on and set it to “Preview in Firefox”. This morning I opened a PDF and it appeared in Adobe instead of the internal viewer. I went back in and saw the application was set back to Adobe. After a couple of quick checks I discovered that Adobe “fixes” (ahem) your settings for you each time it runs. I’m going to poke around in Adobe and see if there is a setting to turn off this “helpful” behavior. I’m not optimistic though.

    1. Martin Brinkmann said on September 11, 2012 at 3:52 pm

      Jim this is surely going to help users with a similar issue. I wonder why Adobe makes those changes without asking the user first. This is like associating all pdf file types on the system with your program every time it is run. Not user friendly.

      1. Jim said on September 17, 2012 at 3:15 pm

        Sorry for the slow response. I went through all the options in both Adobe Reader and Acrobat Pro. I saw no way to turn off this behavior. Pretty much what I expected from Adobe. Adobe Pro inserts itself into every area of your system, so this behavior isn’t a big surprise there. It would be interesting to see if users with just Reader have the same problem.

  2. Tom said on September 10, 2012 at 2:17 pm

    The “Above linked post” points to an article about Thunderbird 15, not Firefox 15.

    1. Martin Brinkmann said on September 10, 2012 at 2:38 pm

      Tom you are right, corrected.

  3. Taomyn said on September 10, 2012 at 9:00 am

    I’m actually using the extension this is based on:


    That way I have the more recent dev versions which so far have been pretty stable

  4. TheRube said on September 9, 2012 at 3:24 am

    . . . OR Try NITRO PDF Reader – – It’s Free; Secure and you can even AFFIX
    your electronic signature to a document

    See Ya!

  5. Redbad said on September 9, 2012 at 12:07 am

    If I could only figure out to disable the annoying “This PDF document might not be displayed correctly” alert…

    1. Martin Brinkmann said on September 9, 2012 at 10:22 am

      Do you have an example for that?

      1. Jim said on September 10, 2012 at 3:29 pm

        I’m getting the same message. It did it on the very first PDF I opened, which happened to be the DropBox “Getting Started.pdf” document. Nothing special that I can see. No forms or anything, just text and pictures. Maybe it’s the pictures?

        (I was using Adobe.)

      2. bill said on September 9, 2012 at 12:09 pm

        I’ve had the same message, think it was after I’d uninstalled Foxit. When Foxit was installed it just seemed to open Foxit in a browser window. With only the Firefox reader I can’t read saved .pdf’s and the Firefox reader and NoScript makes life harder. Reverted to old settings and Foxit.

  6. Redbad said on September 8, 2012 at 11:44 pm

    One might need to go to Tools > Options > Applications and make sure the “Content Type” for PDF “Action” is set to “Preview in ” This action does not appear until that about:config value is false.

    It did not change my “Use SumatraPDF” action. So, I’m thinking the same could be the case for other actions (ask, save, etc.).

    1. Martin Brinkmann said on September 9, 2012 at 10:23 am

      Thanks for the tip. Seems that Firefox does not automatically change the file type association if it is already associated with another pdf reader.

  7. ComicHippo said on September 8, 2012 at 3:50 pm

    Wrong article . Enable “browser.preferences.inContent” for pdf ? WTF !!!

    1. Martin Brinkmann said on September 8, 2012 at 5:00 pm

      You are right, I have corrected the article. Should be correct now.

      1. ComicHippo said on September 8, 2012 at 6:04 pm

        It’s still wrong . You have to set it to pdfjs.disabled to False . Disable False = Not Disabled . Get it ?

      2. Martin Brinkmann said on September 8, 2012 at 6:28 pm

        Right, I overlooked that. Happens when someone else writes the article. It should now be correct.

  8. Fine citizen said on September 8, 2012 at 8:18 pm

    thanks for the tip

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