I did a quick search for "firefox master password" and the first result from mozilla.org mentioned that they recommend using a master password if you share your

I did a quick search for "firefox master password" and the first result from mozilla.org mentioned that they recommend using a master password if you share your computer with someone else. So it sounds like you are right and they only consider it useful for preventing accidental wrong profile use. Edit: Looking through some mozilla bugs starting with the one citrin_ru mentions reminded me that unless they fixed it recently (which I don't think is the case) the master password doesn't encrypt cookies, so it is even less useful for most people than it might seem (and it doesn't normally meet my expectation of helping with a stolen disk).