What's the meaning of this warning from gkrellm?

[Miguel Daniel Rodríguez Magarzo]

Gkrellm notifies that there are new email notifications from root
at /var/log/<myuser>. These mails are generated because of somethings is
wrong with the cron.daily, but I don't know how to interpret it:

---------------------------------------------------------
>From root at ulises Sun Feb 10 12:09:33 2008
Return-path: <root at ulises>
Envelope-to: root at ulises
Delivery-date: Sun, 10 Feb 2008 12:09:33 +0100
Received: from root by Ulises with local (Exim 4.67)
	(envelope-from <root at ulises>)
	id 1JOA3w-00073n-OM
	for root at ulises; Sun, 10 Feb 2008 12:09:33 +0100
From: Anacron <root at ulises>
To: root at ulises
Subject: Anacron job 'cron.daily' on Ulises
Message-Id: <E1JOA3w-00073n-OM at Ulises>
Date: Sun, 10 Feb 2008 12:09:32 +0100

/etc/cron.daily/logrotate:
error: firebird1.5:8 unknown user 'firebird'
run-parts: /etc/cron.daily/logrotate exited with return code 1
--------------------------------------------------------------------

A search with Synaptic says that something called firebird is installed
in this system,that's a kind of SQL database, probably installed as
dependency of other package.


............................................
common files for firebird 1.5 servers and clients
This package contains files common to both servers and clients for both
super
and classic flavours.

These include the security database, message file (firebird.msg),
support
scripts as well as configuration files in /etc/firebird/1.5.

Please refer to the firebird1.5-super or firebird1.5-classic packages
for more
information about Firebird in general.

 Homepage: http://www.firebirdsql.org/
............................................................................

How to avoid that messages from gkrellm? Anyone knows, please?
Actually, it doesn't matter much IMO, but I'd like to be able to do that
they disappear. 

Daniel