[Cpoll]

This sounds a bit like an extension that nests tabs in a tree structure (I can't remember the name).

[alasarmas]

Tree Style Tabs?

https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...

[dvogel]

Both TST and Sidebery are leagues ahead of tab groups IMHO. Frankly sidebar tabs just work better for 16:9 aspect ratio because with a maximized window most sites don't make good use of 1/4 of the width anyway.