If you are not already aware of this trick, I am sure the title of this post would confuse you but that is exactly what this trick is all about – running Firefox inside Firefox
Well, I am not sure if this trick has any practical usage other than that you can show it off to your friends.
Just open Firefox and in the address bar type the following url.
If you are already reading this post in Firefox, then you could as well click on the above link to start a Firefox browser inside your already running Firefox.
You could again enter the above url in the second Firefox that appears to open a third one. You could repeat this as many times as your screen size permits.