I actually use separate windows for this purpose. Tabs have been draggable between windows for a while now. Drag the tab off, you have a new 'group'. The system task bar keeps an ordering, so I have spacial recognition.
I discovered tree-style tabs a few months ago and love them. However, I find myself running Chrome for its superior performance on my Mac. Is there a grouping tab manager anyone can recommend for Chrome?
Also, at one point, I was curious, so installed this plugin so I could stop counting... https://addons.mozilla.org/en-US/firefox/addon/4422/
There's also tree-style-tabs... https://addons.mozilla.org/en-US/firefox/addon/5890/