When you initially obtained your certificates, you were likely
prompted to set a password to protect them.  Firefox refers to this
password as the "master password for the Software Security Device".

This password is used to protect your certificates and any website
login information you may have asked Firefox to remember.  It is
completely unrelated to your Kerberos password that you use to log in
to Athena and check e-mail.

If you have forgotten the password, or are being repeatedly prompted
for the password, please see our stock answers on that topic in this
category.