Attention: Firefox 15+ Private Browsing mode broken

Martin Brinkmann
Sep 5, 2012
Updated • Sep 7, 2012

Firefox's private browsing mode is a special mode of the browser where no information about visited websites are recorded in the browser or on the computer. That's great for a variety of purposes, from shopping for presents to accessing contents on the Internet that you do not want other household members to know about.

The recent update to Firefox 15 introduced a bug in the browser's private browsing mode that has been filed on Bugzilla a few days ago. It is still possible to switch to the browser's private browsing mode, but doing so will not block the generation of cached files while you are in that mode, which in turn reveals information about the visited websites and contents after exiting the mode. That's exactly the opposite what the mode has been created for, and such a big issue, that Mozilla is already preparing a patch for all supported versions of the browser.

The issue affects all release channels of the browser: stable, beta, aurora and nightly. You can reproduce the issue by clearing the browser cache, starting private browsing mode, navigating to one or two websites that you can identify later, and opening about:cache to check the cache entries under disk cache. You should see cached items while in private browsing mode, but also after ending private browsing mode.

Mozilla is already preparing Firefox 15.0.1 candidate builds and it is very likely that we will see a Firefox 15.0.1 release in the next couple days.

If you use private browsing mode regularly you may want to make sure to delete the browsing history manually in the browser whenever you exit the mode. Since this impacts your regular browsing history as well, it may make more sense to use a different browser or a portable version of Firefox for that in the meantime. Last but not least, it is also possible to downgrade Firefox to version 14.x instead until the issue is resolved. (via FF Extension Guru)

Update: Firefox 15.0.1 has been released and is now available on the Mozilla website and via the browser's automatic update feature. A Mozilla spokesperson provided me with the following statement.

Yesterday we released an update to Firefox for Windows, Mac and Linux to address an issue that caused some Web page content to be stored in the Firefox cache while in Private Browsing mode. At no point was the data stored in cookies or the user's history. As always, Mozilla did not capture any personal data.

This refers to the deleting the browsing history part of this article. What I meant to say was to make sure the cache gets deleted when you use the delete browsing history feature of the browser.


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  1. pd said on September 6, 2012 at 6:50 pm

    piss poor effort. how many channels and pttesters do they need before cock-ups like this stop?

  2. Fox said on September 6, 2012 at 5:38 pm

    Strange, but I didnt experience this bug neither in Fx15 nor in 16b which Im using now, hmm..

  3. Jonas said on September 6, 2012 at 1:03 pm

    I thought they had automated build tests in place to check against these things.

  4. senina said on September 6, 2012 at 7:55 am

    I have used firefox for a long time but never used the private mode. Love its stability and various add-ons. I often firefox, chrome and Avant browser these three at the same time. Different browser visits different webpages.and they each have some useful features I like. I like the password manager of firefox, chrome’s fast speed and Avant’s detach tab and three engines. They are good combination.

  5. Richard Steven Hack said on September 6, 2012 at 6:03 am

    I just love Mozilla’s QA process… It’s non-existent… In ANY COMPETENT QA using regression tests, this bug would have been caught…

  6. i say said on September 6, 2012 at 2:38 am

    i hope firefox will replace private browsing with a built-in vpn. that should do it.

  7. anony said on September 6, 2012 at 1:34 am

    Great, and they still have the most retarded private browsing mode too. Why does it close non private window when I enter one? Why can’t it be like Opera or even Chrome?

    I mainly use private mode on Google search, since I’m constantly logged on if I don’t use private (and logging out only to automatically logged in if I want to use any of their service get tiring fast).

  8. Samuel Becks said on September 5, 2012 at 11:35 pm

    Yeah this is a big oversight. However such mistakes do occur – so long as they are fixed and are an uncommon occurrence then its not so bad. As long as there wasn’t a bad intention behind it…….

  9. Jeremy Collake said on September 5, 2012 at 10:37 pm

    I am not one to be too harsh on those that make mistakes, but as far as mistakes go, this is a huge one. I mean, the ‘evidence’ (of whatever) that the person hopes to avoid leaving by using this mode, is indeed left. It could cause some people substantial complications at home or work, assuming they have some reason for using this mode, and are relying on its word that it doesn’t leave trace evidence on the PC.

  10. DanTe said on September 5, 2012 at 10:28 pm

    Thanks for the heads up. This is a doozy.

  11. Karl Gephart said on September 5, 2012 at 9:19 pm

    Another option is the handy add-on, Private Browsing Windows, which still works great in Firefox 15. Opens a separate window, not a tab.

    1. Martin Brinkmann said on September 5, 2012 at 9:25 pm

      Have you verified that Firefox is not storing cache entries while in private browsing mode? Remember that it makes use of a second profile and that you need to check the cache of that profile to verify.

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