Firefox: save username/password for all sub domains?

Not sure if this is possible -- I have a username and password combo which is valid for all sub-domains of a given domain, that is: domain1.example.com, domain2.example.com, domain3.example.com, etc. Is there any way I can tell Firefox that the saved password for one of these subdomains is valid for all of them? RIght now I have been retyping it for each one, and saving them individually. But the list of these subdomains will be constantly growing...