A post on the Debian forum alerted me to the fact that Debian is tracking the Firefox Extended Support Release: my Debian Testing installation comes with Iceweasel 10.0.3esr-3. I blogged about the ESR release here. It provides security updates for longer, but feature updates less often than the normal release: Iceweasel 10 ESR has the security updates of Firefox 12, but without the new features.
Debian packages are ultimately progressing towards the Stable release, with long term support, so it obviously makes sense for Debian to track the ESR release, but ironically it means that my Testing installation uses Iceweasel 10 while my Stable installation uses Iceweasel 12 from the Debian Mozilla team.
Currently there are no obvious feature differences between Iceweasel 10 and Firefox 12, but Firefox could reach version 18 before Iceweasel sees a feature release.
(Graphic from Mozilla Firefox ESR Overview.)