You're probably getting "no package available" because the version you've specified isn't actually available in the stable (Lenny) repository, and your
sources.list file probably doesn't include the oldstable (Etch) repository.
But I wouldn't add the old repository immediately. Instead, download the Etch binary from the link and try to install it manually:
$ sudo dpkg -i iceweasel_18.104.22.168-0etch1_i386.deb
If you run into dependency problems, purge that package (
sudo apt-get purge iceweasel) and make a choice:
Enable the oldstable repository and install with aptitude, which might downgrade other packages while attempting to satisfy dependencies; or
Grab the sources (also available from the Etch link, in the right-hand sidebar), install the build prerequisites, and compile your own version of the package.
Whichever route you decide on, your "Hold" should now work.