Chrome supports multiple users pretty well, but it's per-window not per-tab. (I think that might be better anyway, it seems like it would be easy to lose track of which tab is logged into what)
You can use multiple profiles in Firefox. There is even an addon to make the switch easy: https://addons.mozilla.org/en-us/firefox/addon/profileswitch....
It's the simultaneous different 'users' (ie, cookie stores, mostly) in different windows that is exciting to me.
It lets you have different users in different windows simultaneously, or it makes you switch the entire environment all together?
Thanks!