It would be good to clearly document the location where the master password is stored, if the user selects to have it stored to disk. This way, the user could conveniently instruct the operating system to encrypt this file.
As you probably know, OS-level encryption is much stronger than what the extension can do by itself, since it is keyed off the password used to log into the workstation.
Even better would be for the extension to do this, but I suspect that Firefox would need to provide core support for such a feature.