Você pode usar /etc/apt/preferences
para especificar quais versões você deseja por pacote.
Para ter a versão mais recente do iceweasel, use (por sua conta e risco) os seguintes arquivos de configuração:
/etc/apt/preferences
Package: iceweasel
Pin: release a=experimental
Pin-Priority: 1000
Package: *
Pin: release a=testing
Pin-Priority: 500
Package: *
Pin: release a=unstable
Pin-Priority: 400
Package: *
Pin: release a=experimental
Pin-Priority: 300
/etc/apt/source.list
deb http://http.debian.net/debian testing main contrib non-free
deb http://security.debian.org/ testing/updates main
deb http://http.debian.net/debian unstable main contrib non-free
deb http://http.debian.net/debian experimental main contrib
Mas cuidado, pois o apt_preferences (5) página de manual avisa
Packages included in a specific release aren't tested in (and therefore don't always work as expected in) older or newer releases, or together with other packages from different releases. You have been warned.