Very carefully. Unlike other browsers, it is all or nothing, it is either
on for everything or off for everything. You might actually be better
off using two different profiles. Accessible from the Tools menu
"Start Private Browsing" or it's shortcut listed there.
http://support.mozilla.com/en-US/kb/private+browsing
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HTH,
David McRitchie, extensions I use are briefly documented on my site
Firefox Custom: http://www.mvps.org/dmcritchie/firefox/firefox.htm