I just want to show you a fun thing to do with firefox 4.
so just enter "chrome://browser/content/browser.xul" as URL and press enter. this creates another browser in the current tab.
Of course you can "go deeper"
It's not shopped! try it out yourself.
I could imagine to use it as categorizer in my browsing, but unfortunately firefox will NOT save tabs inside those tabs, so once you accidently hit close it'll be all gone.
Oh well... maybe in the next ff update.