Master passwords have a very narrow threat model that they are actually protecting against: it's protecting against people who have access to the password database (i.e., filesystem access) but not people who have access to ptrace-privileges (i.e., the ability to read the password directly out of process memory). Which pretty much means the only realistic use case is someone stole your laptop, but full-disk encryption is a much more relevant solution.

Mozilla has known for a very long time that the master password system isn't very effective. Even a decade ago, I remember them recommending not using it.