Try this , hope it wil help you :
Here is a simple extension to disable the "Tools -> Start PrivateBrowsing" menu entry and the "Tools > Clear Recent History" menuentry in Firefox 3.5/6.
The extension hides these menu entries and also disables their shortcutkeys and should prevent Firefox from entering Private Browsing mode.The CSS code (chrome://pbdisable/content/pbdisable.css) in theextension also hides "Tools -> Options
-> Privacy -> Automatically start Firefox in a private browsingsession" and the History setting. You can copy that code to userChrome.css
if you want these entries hidden if the extension is disabled. CurrentFirefox versions still process code in userChrome.css if started in Safe_mode
.Keep in mind that removing or disabling "Private Browsing" will notmake it impossible to delete private data like visited websites(history) or cache. It is possible to clear the cache via "Tools -> Options
-> Advanced -> Network" and to clear history in the Library and History sidebar.
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