GPG Error while apt-get update

Sat Jul 20 06:38:07 UTC 2013

Hello. I get this error while 'apt-get update' :
"GPG error: http://mozilla.debian.net wheezy-backports Release: The
following signatures were invalid: BADSIG 85A3D26506C4AE2A Debian Mozilla
team APT archive <pkg-mozilla-maintainers at lists.alioth.debian.org>"
debian-keyring, pkg-mozilla-archive-keyring packages have been installed.
In what can be the problem?
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