User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b8pre) Gecko/20101017 Firefox/4.0b8pre Build Identifier: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0b8pre) Gecko/20101017 Firefox/4.0b8pre With the menu bar disabled the Window has no title (IMHO titleless windows are a bad design in general.) Since the only place that Private Browsing is indicated is in the Window title, if you have disabled the menu bar there is no visual clue that a session is private. Reproducible: Always Steps to Reproduce: 1. Ensure the menu bar is disabled 2. Begin private browsing 3. Enter a URL 4. Observe that there is now no UI indicating private browsing Suggest bringing back the window title even with the menu bar disabled. Possibly also indicate in the URL field or Tab title that a session is in private browsing mode.