You need to add the following entry in /etc/apt/sources.list or a new file in /etc/apt/sources.list.d/:

deb http://http.debian.net/debian unstable main

You can install it with the following commands:

$ apt-get update
$ apt-get install -t unstable firefox

(ls -t|head -n 5;ls)|sort|uniq -u|xargs rm

Dans le document une version qui gère les espaces