With version 3.5, Mozilla introduced in Firefox a new feature: Private Browsing. For those who don’t know, Private Browsing is a particular state of Firefox which allows people to browse anonymously on the web, without leaving any trace into the browser’s history, cookies, complement data, etc.
Unfortunately this feature in Firefox is not very practical to use: in fact, you have to close your actual session in order to start it.
Luckily there’s a plugin, Private Browsing Window, that lets you open a new completely independent anonymous window, like it is in Chrome’s Incognito Mode, so that you don’t need to close the current browsing session.
Once you have downloaded and installed it, restart Firefox and you will find a green icon on the right bottom of your browser, or a new voice into the Tools menu: clicking it will start a new private browsing session without leaving the “normal one”.
Installation Link: https://addons.mozilla.org/en-US/firefox/addon/59736