Well you can delete it using this add-on https://addons.mozilla.org/en-US/firefox/addon/menu-editor/ Install it, restart firefox, go to tools -> addons -> menu editor options. Select 'Tools' from the dropdown box below 'Edit Menu'. Scroll down to Private Browsing, click on the green colored tick mark next to it, and you're done. However, it can still be accessed by typing about:privatebrowsing in the address bar. To disable, To permanently disable private browsing in Firefox: IMPORTANT: This alteration MAY effect the way Firefox automatically restores tabs and may have potential effects on your Firefox Add-ons. 1. First, ensure that the program is closed. 2. Go to your windows drive (usually drive C), browse to "/Program Files/Mozilla Firefox/components/". 3. Delete the file "nsPrivateBrowsingService.js".

I just published an extension to disable private browsing here. Do note that it is trivial for a savvy user to disable this add-on. But it might help some users in some cases. Please give feedback on the add-on page if the add-on does or does not work for you.