This basically means that it can be that date gets stored on the user’s computer in private browsing mode if an add-on has a function to store data.

That’s obviously not the intention of the private browsing mode and the Mozilla developers have addressed the inconsistency in a blog post.

The developers, once again, explain that private browsing mode has been designed to only prevent browsing data to be stored on the computer system. Browsing data refers to data that can be used to identify the websites the user was on. This includes website addresses, cookies or temporary data.

The developers have agreed on how privacy support in add-ons has to be realized:

• If your add-on stores browsing data in any way, it must support PBM. This support cannot be disabled in any way, not even with hidden preferences.
• If your add-on stores some other type of personal data, support for PBM is optional. What we did with Fire.fm is a good guideline: have a preference “support PBM for the data this add-on handles”, turned on by default.

Mozilla will start enforcing this new rule at the end of March. Add-ons that are updated or newly added to the Firefox add-on repository after that date will be rejected if they do not comply with the new privacy rules. The rules will however have no effect on add-ons already listed in the extension gallery. (Mozilla blog via Techie Buzz)

This behavior is changed by the Firefox add-on Private Browsing Window which – as the name suggests – provides the means to launch a private browsing session in a new window without closing the standard session first.

The extension adds a Private Browsing Window link to the Firefox Tools menu and to the status bar of the browser. A click on the link or button will launch a new Firefox window in private browsing mode. The extension will display an add-on installation dialog upon first usage that tries to install the Private Browsing Window Helper extension. This extension is not required for the core functionality. It does however improve interaction between non-private and private windows. It specifically adds an option to open links in the non-private or private browsing windows.

Private Browsing Window makes it a tad easier to work with the private browsing mode in Firefox. The extension is available for Firefox 3.6 only and can be downloaded from the official Mozilla Firefox add-on gallery website.

Update: The Firefox add-on has been discontinued as it is not necessary anymore. The Private Browsing Mode can be launched via the Tools > Start Private Browsing menu or with the keyboard shortcut Ctrl-Shift-p.

Most web browsers come with a switch to enable private browsing mode once the web browser has been started. The option in the Firefox web browser is available under Tools > Start Private Browsing or by pressing the Ctrl Shift P keyboard shortcut.

This can be a nuisance for some Firefox users who prefer to always browse the Internet in private browsing mode. This can for instance be interesting for users who use a portable version of Firefox on an USB stick to prevent that data gets stored on the system they use Firefox on.

Firefox comes with a configuration switch since version 3.1 which can be set to true to turn on private browsing mode automatically whenever the web browser is started. The following steps need to be performed to autostart Firefox in private browsing mode:

• Enter about:config in the Firefox address bar. If that is the first time click on the I’ll be careful button.
• Paste browser.privatebrowsing.autostart in the form at the top to filter for the entry needed.
• Double-click the entry. This will automatically change the value from false to true.

Firefox will start up in private browsing mode on the next start of the browser. The changes can be undone by performing the same steps that are outlined above.

It is important to note that private browsing is only helpful to avoid records of browsing sessions on the computer used. It does not hide the user from the Internet Provider or websites that they visit.

Private Browsing can be enabled in the Firefox Tool menu. It will display a popup that contains information that private browsing mode will be started and all currently open tabs will be saved to be reopened after the private browsing session ends.

Firefox will not record browsing history, search history, download history, web form history, cookies and temporary Internet files. Bookmarks as well as downloads on the other hand will be retained in private browsing mode.

Firefox will add the (private browsing) string to the title which is a indicator for anyone that Private Browsing is enabled. Private Browsing can be disabled from the Tools menu again by selecting the same menu entry that activated it.

Users who regularly clear data when closing Firefox might wonder why they should be using the Private Browsing mode at all. The main difference between clearing data when closing the browser and using the Private Browsing mode is that the Private Browsing mode will never store data on the hard drive. This might however happen when cleaning Firefox data by closing the browser. First, data could be recovered and second it will still be there if Firefox closes unexpectedly.