I have set up a master password on firefox and the browser asks for it at unexpected time. Not every time in the start (so it isn't useful to "lock" the browser. Someone may auto-login to a website without typing the master password), just at "random" time.

I don't really care about "locking" my browser, I just want to protect my set of passwords from unauthorised view. How can I make firefox ask for the master password ONLY when trying to show passwords?

Changing security.password_lifetime to a large value didn't work as a semi-solution, because it will ask again at random time if I restart firefox. I would also like to avoid using a separate password manager software.