I'm pretty sure the profile manager stuff in Firefox goes all the way back to the Netscape 4 days. If I recall correctly, back in the way back, it used to throw you into Profile Manager and walk you through creating your initial one on first launch of fresh installs. At some point they tucked that under the rug and just started creating the default profile automatically - maybe because it made less sense to ask for a bunch of personal information up-front when the browser was broken off from the mail client? Now most people have forgotten that Profile Manager even exists, but it's been there the whole time.
my parents rely on Profile Manager. I set that up for them years ago, since my mom uses my dad's computer a bit every day. She has no need for a whole user account. Separating the stuff in the browser is enough.
