When you initially obtained your certificates, you were likely prompted to set a password to protect them. Firefox (and Thunderbird) 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 email.
If you have forgotten the password, or are being repeatedly prompted for the password, please see our stock answers on master passwords.