Firefox’s “Private Browsing” mode is a mode in which you can use Firefox to surf the web without Firefox storing any data on where you have been, what you did, etc. In other words, Private Browsing is supposed to make it so you leave no trace on your computer of your Internet activity. (Emphasis on “on your computer” because Private Browsing does not, and cannot, restrict servers from gathering data on you — it only controls what is stored locally on your computer.) Or, at least, that is what it is supposed to do in theory. According to a bug report filed in Bugzilla, Private Browsing mode in Firefox 15 and higher has a bug: Firefox’s cache retains data on websites visited while in Private Browsing mode after Firefox has been closed.

This bug, if proven true, renders Private Browsing mode to be useless because the whole point of Private Browsing is to not leave any local traces of your activity once you exit Private Browsing. Reading through the comments on the bug report, it seems like a few people have been able to reproduce the bug on Firefox 15 and higher, and Martin Brinkmann of Ghacks corroborates with his own experience confirming the issue.

Personally speaking, when I tried to reproduce the bug on Firefox 15 I ran into something odd. While using my regular Firefox profile, which has a few add-ons installed, I was able to reproduce the bug. However, when I went to a completely new and clean Firefox profile, I was unable to reproduce the bug. In other words, during my quick tests, a new Firefox profile resulted in data being properly flushed after exiting Private Browsing mode whereas using my regular Firefox profile resulted in some (but not all) data being retained after exiting Private Browsing mode. This could mean that the bug is being caused by add-ons as opposed to Firefox. However, I cannot confirm or deny this theory.

Anyone that wants to test this bug on their own can do the following:

• Close all open websites.
• Clear all browsing history.
• Type about:cache in your address bar and hit enter.
• Once you are at about:cache page, confirm nothing is stored in your cache.
• Open Private Browsing mode.
• Surf a few websites.
• Close Private Browsing mode.
• Go to about:cache and see if any data from your Private Browsing session is stored in Firefox’s cache.

Mozilla is already prepping Firefox 15.0.1 for release so this Private Browsing bug could be fixed in that release although Aurora builds of Firefox 16 and higher seem to still have it (according to user comments on the bug report), so don’t get your hopes high. For now, if you are a regular user of Private Browsing think about either switching to a different browser or remembering to manually clear your Firefox history every time before exiting.

[via Ghacks]