Re: I want to remove the "private browsing" option. how?
You have or had an extension installed (NET_mmhpset) that has changed the user agent from Firefox/3.6 to Firefox/3.0.x.
You can check the general.useragent prefs on the about:config page.
You can open the about:config page via the location bar, just like you open a website.
If general.useragent prefs are bold (user set) then you can right-click that pref and choose Reset.