Some research led me to the belief that the only real option was to reset the master password - and destroy all the saved passwords with it. I considered using crack tools to determine the password, but first decided to try doing the password reset. I still had the passwords in the old Netscape profile, which I could re-import if I had to. I performed the password reset using this in the Firefox address bar: chrome://pippki/content/resetpassword.xul . The outcome was surprising: the browser closed itself, and when I opened it up again and went to a login page with a previously-remembered login, my login details were still there! Furthermore, in the Privacy:Passwords section of Firefox options, there was no master password set! I could still see all my login details without hindrance, and now I could add new ones! Hurrah!
This password reset did the job for me, but it may not work for you. I cannot be held responsible for any loss of data, income, hair, etc as a result of you copying what I did here. You're on your own!