I just came across this nifty little Firefox trick that is sure to drive some of your friends crazy! As seen in the screenshot above you can very easily embed multiple Firefox windows inside of your main Firefox window. All you have to do is enter this into the Firefox address bar and press Enter:
chrome://browser/content/browser.xul
The usefulness of this trick is probably next to nothing, but it will surely drive your friends crazy when they see their Firefox looking like the screenshot above! If you set their homepage to the address listed above it will automatically create a second Firefox window, but from what I can tell you can’t make it create multiple windows inside of windows like I did above without manually typing in the address into each address field.
Make sure you let us know any good tricks you pull with this one. 
Source: SeeJay
That was odd. Kinda interesting, but completely useless. It also wouldn’t work for me more than once. By that I mean I could only make one extra toolbar appear. I wonder why Mozilla put this in there?
Session manager actually tells me that my session has crashed once I enter in that URL and it offers to open my tabs that were open in the main window.
This might be stretching it, but I may have found a use for this. If you find yourself opening countless tabs, you could try organizing them by putting them within (chrome…browser) root tabs. Like tab groups or sub-tabs.
Hmmm, wonder why you could only get it to open once? I kept pasting it in the address bars that were appearing and it kept replicating itself.
Wow, I didn’t actually think about that at the time because my tab bar was hidden but that is actually kinda nifty.
old old news…
[kb.mozillazine.org]
[kb.mozillazine.org]
Of course it is old news that it can be done, but I just don’t think people ever really thought to try it. I’m sure this could have been done for quite some time with Firefox.