Any browser that can let a user switch between Google or Yahoo or Facebook accounts quickly?

Firefox supports Profiles, also there's CookieSwap extension that makes it possible to use different accounts with a single profile (though not a the same time).

Launch Firefox with the -profilemanager option, then you can create additional profiles you can use with the -p "profilename" command line switch (you could make a shortcut). You couldn't use them in the same window, but you could use them simultaniously. You could install an extension to change the window title to differentiate them more easily.