Interesting. I've been using the Tree Style Tab addon, which seems to achieve the same goal of grouping and nesting with less administrative overhead:

https://addons.mozilla.org/en-US/firefox/addon/5890/

don't try Tree Style Tabs; ones hooked and you can never go back :)

No seriously, Tree Style Tab is the best experience I ever had as a notorious tab user.

"once hooked"

Not as pretty/fancy, but possibly more intuitive.

true, looks smooth. demo-video: http://lifehacker.com/5595027/firefox-tab-candy-organizes-yo...

aids the cognitive map you have of your tabs - much better than FF's default tab-scrolling.