Although I'm close to ambidextrous, I have yet to manage the art of actively browsing the web while actively switching folders in the mail clients. Maybe if I had two keyboards attached to this machine it'd work, but I don't really feel the urge to try.

In other words, while theoretically a problem, in practice I don't see this as a viable reason to refrain from using both mail and browser in the same process space, ie. Seamonkey, not even on a relatively under-powered (1.8GHz Pentium M) machine.