I've not actually used Private Browsing, but I found what looks like a great support article (starting place anyway) from Mozilla:
http://support.mozilla.com/en-US/kb/Private%20Browsing#w_how-do-i-turn-on-private-browsing
Ginger Stoneton, Technology Integrator
Maine Regional School Unit 24
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Is there a way to disable to the private browsing feature in
Firefox.....HELP!
Lori Stevens