You can add the following two lines of code to userChrome.css, this will remove the Private Browsing options from the menus. This code needs to go below the line that begins with @namespace. For details on this file including its location, see http://kb.mozillazine.org/UserChrome.css

#sanitizeSeparator, #privateBrowsingItem, #sanitizeItem {display:none!important;}
#privateBrowsingAutoStart {display:none!important;}

For details on editing configuration files such as userChrome.css, see http://kb.mozillazine.org/Editing_configuration

You can still use the keyboard shortcut (Command+Shift+P) to toggle private browsing.

That userChrome.css code probably won't work on a Mac because not Firefox but the OS maintains the menu bar.

Thanks anyway...

I don't think this addressed the problem or at least in language I can understand. Please don't assume we are all computer experts. Could you give more basic instructions for this solution. What and where is userChrome.css?

TonyE provided a link that explains userChrome.css
http://kb.mozillazine.org/UserChrome.css

You also need to read this about editing those types of files.
http://kb.mozillazine.org/Editing_configuration

That userchrome.css code worked in FF 3, but doesn't appear to work in FF 5. I have tried taking the same userchrome.css file that killed Private Browsing in FF 3 and placing it in the \<user>\AppData\Roaming\Mozilla\Firefox\Profiles\<profile name>\chrome directory (using a Win 7 machine) but either FF 5 doesn't recognize it or the CSS has changed because of the new interface.

Does anyone have any suggestions?

@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");

/* Hide Private Browsing in App menu and Tools menu */
#appmenu_privateBrowsing, #privateBrowsingItem  {display:none!important;}

/* Hide Sanitize item in App menu and Tools menu */
#appmenu_sanitizeHistory, #sanitizeSeparator, #sanitizeItem {display:none!important;}

This extension disables Private Browsing, hides all the PB menu items and disables the keyboard controls, disables the clearing of browsing history, and the extension doesn't appear in the Add-ons except for the Firefox SafeMode.
https://addons.mozilla.org/en-US/firefox/addon/disable-private-browsing/

This thread was originally about only removing Private Browsing from the menus and not to disable that feature completely, because the OP still wanted to use PB mode for visiting some websites.

but want to be able to use it for banking etc. I really just don't want it to be available from Tools

Yes it was, but it has been revived twice. I posted about an extension that was just released, in case anyone wants to block PB.