Firefox 3.1: Automatically Start in Private Browsing Mode
Contributed by Rob Rogers 
Tagged: Mozilla Firefox Private Browsing allows you to surf the web without recording a history of the sites visited and without saving any information about the session. If Firefox is to be installed on a public computer, then starting it in Private Browsing mode is a must.
1. Open Firefox 3.1.
2. In the Location Bar, input about:config.
3. Click to get past the warning message that appears.
4. Locate the Preference Name browser.privatebrowsing.autostart.
5. Double-click the preference and change the value to true.
The next time you open Firefox, it will be in Private Browsing mode.
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JC said on November 27, 2008
by any chance, would this also work for any application data information, such as flashplayer cache and saved object? Flash and other ‘equivalent’ 3rd part apps presents in web page store a lot of information… Looking at the Application Data folder, it is possible basically to see the list of web sites currently looked at, as most of them use flash…
fastfreddie1959 said on November 27, 2008
In mine it did not exsist.
Had to right click-new-Bootlean-browser.privatebrowsing.autostart.–and put in true.
Manu said on December 3, 2008
“browser.privatebrowsing.autostart” .. it is not present as an option..
FF said on December 5, 2008
No option as browser.privatebrowsing.autostart on mine… i’m running the latest version.
reelz said on January 15, 2009
this is for 3.1 beta peeps
TanelTM said on January 13, 2011
How about having a command-line option for that?
Me said on December 27, 2011
This totally did not work for me… I restarted firefox (latest version) and it re-opened in normal browsing mode.