Mozilla could use a '-noserver' option (or the same functionality by some other means) to complement the '-server' option that would unload Mozilla from the background and free-up RAM without a reboot. especially useful if you use the '-server' option with Moz in your Windows 'startup' folder (related to bug 75599)
"mozilla -kill" does what you want, I think. It turns off "server mode" and closes any "cached" Nav window. Doesn't kill the process outright, however.