I think this is the reason:
Please remember that modifications to core files like are done at your own risk - there's a reason the Mozilla development team has structured and compressed these files as they have, and re-enabling the auto-saving of passwords on certain sites could pose a security threat.
from a site
I found, but, it seems to be possible, info can be found there too...
(BTW - I never allow any passwords to be remembered on my PC, just to avoid loss of a password. But, that's just me)