To install iceweasel 3.5 in debian lenny you need to do the following steps:
Iceweasel 3.5 installation on debian lenny
1. Add the debian repositories testing and experimental:
# echo deb http://ftp.fr.debian.org/debian testing main > /etc/apt/sources.list.d/testing.list
# echo deb http://ftp.fr.debian.org/debian experimental main > /etc/apt/sources.list.d/experimental.list
2. Leaving the stable distribution on by default:
# echo ‘APT::Default-Release “stable”;’ > /etc/apt/apt.conf.d/default
3. Update repositories:
# aptitude update
4. Install iceweasel 3.5 units
# aptitude install -t testing libstartup-notification0 libsqlite3-0 libnss3-1d libhunspell-1.2-0 libasound2
5. Install iceweasel 3.5:
# aptitude install iceweasel/experimental