Xulrunner provides a simple password management service. To use it, put the following in your rc:
session_pref("signon.rememberSignons", true);
session_pref("signon.expireMasterPassword", false);
session_pref("signon.SignonFileName", "signons.txt");
Components.classes["@mozilla.org/login-manager;1"]
.getService(Components.interfaces.nsILoginManager);