# apt-cache policy iceweasel
iceweasel:
Installed: 12.0-7
Candidate: 14.0~a2+20120601042008-1
Version table:
14.0~a2+20120601042008-1 0
500
http://mozilla.debian.net/ experimental/iceweasel-aurora i386 Packages
13.0~b6-1 0
500
http://mozilla.debian.net/ experimental/iceweasel-beta i386 Packages
*** 12.0-7 0
1
http://ftp-mirror.internap.com/pub/debian/ experimental/main i386 Packages
100 /var/lib/dpkg/status
10.0.4esr-3 0
500
http://ftp-mirror.internap.com/pub/debian/ unstable/main i386 Packages
Firefox 13 was released today.
Mozilla/5.0 (X11; Linux i686; rv:13.0) Gecko/20100101 Firefox/13.0
and in order to get Iceweasel 12 on Wheezy or unstable you have to install it from experimental.
Firefox 12 automatically downloaded the upgrade to 13. Starting with Firefox 14 it will upgrade automatically all on its own.