When setting up Ubuntu, I usually add in Microsoft’s core fonts by installing the msttcorefonts package using either Synaptic or apt-get. I am not sure why I didn’t try doing the same thing for Debian until now but it’s equally as feasible. Just pop over to System > Administration > Software Sources and ensure that the check-boxes for the contrib and non-free categories are checked like you see below.
You could also achieve the same end by editing /etc/apt/sources.list and adding the non-free and contrib keywords to make lines look like these before issuing the command apt-get update as root:
deb http://ftp.debian.org/debian/ lenny main non-free contrib
deb-src http://ftp.debian.org/debian/ lenny main non-free contrib
All that you are doing with the manual editing route is performing the same operations that the more friendly front end would do for you anyway. After that, it’s a case of going with the installation method of your choice and restarting Firefox or Iceweasel to see the results.