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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