Disable private browsing Rated 3 out of 5 stars

The following worked for latest version of Firefox. Will hide the menu options for private browsing, but probably not actually disable that option. (Thanks to cor-el and the post found at https://support.mozilla.org/en-US/questions/768275)
Some of the older advice found in a web search works only if View Menubar is switched on, but then doesn't work if View Menubar is off. The following works for both cases.

First you need to edit the file called userChrome.css
This file will be located in your profile folder, described here. (This is for Windows Vista; would likely be different for other versions of Windows.)

C:\Users\<Your Account Name Here>\AppData\Roaming\Mozilla\Firefox\Profiles\<random number>.default\chrome

To find this folder you would have to show hidden folders. Open Windows File Explorer and get access to the menus by hitting the Alt key. Then select Tools and then Folder Options, then click the View tab, then click Show Hidden files and Folders, then click Ok.

Then once inside that file, find the line with @namespace.

Just after that line, paste the following.

/* 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 review is for a previous version of the add-on (1.7.1-signed). 

This addon does all this, and more. Is there anything in particular you still see/need after installing this addon?