yum doesn't remove old package files

Hi,
a proof of $Subject:

[nico@nico2 ~]$ rpm -qa | grep -i firefox
firefox-2.0.0.9-1.fc8
firefox-2.0.0.9-1.fc8

[root@nico2 ~]# du -sh /usr/lib/firefox-2.0.0.*
6,8M    /usr/lib/firefox-2.0.0.6
20K     /usr/lib/firefox-2.0.0.8
43M     /usr/lib/firefox-2.0.0.9

Obviously
rpm -qif on /usr/lib/firefox-2.0.0.6 and on /usr/lib/firefox-2.0.0.8
reported that the directories weren't owned by any package.
Same problem with kernel modules (that occupied a much
larger amount of space).
Why aren't these files removed? I run yum clean all regularly.
Is there  a way to purge entirely old packages?

Thanks,
	Nico