There is no Firefox package in Debian. You can read about it here: http://en.wikipedia.org/wiki/Mozilla_Corporation_software_rebranded_by_the_Debian_project
If you found this article, it means that you tried to install Pencil from officially provided deb-package (today it is http://code.google.com/p/evoluspencil/downloads/detail?name=evoluspencil_2.0.5_all.deb&can=2&q=label%3A2.0.5+OpSys-Linux).
Generally, dpkg can not resolve firefox-dependency. I do not see any advantages of Firefox over Iceweasel. Keep in mind, that you can have only one of that applications — Firefox or Iceweasel. So I don’t want to install Firefox. Instead, I updated my Iceweasel guided by http://mozilla.debian.net/:
You need to add the following entry in
/etc/apt/sources.listor a new file in
$ apt-get update
$ apt-get install -t experimental iceweasel
After that, I avoid firefox dependency in Pencil by simple swapping
iceweasel in deb-package next way (as described here http://forums.debian.net/viewtopic.php?f=6&t=106325):
Basically what you have to do is extract the files from the deb
dpkg-deb -R file.deb temp-dir
DEBIAN/controland remove the
firefoxoccurrence from the dependency list.
After that you repackage the deb
dpkg-deb -b temp-dir file-mod.deb
Once you’ve done that the installation should go smoothly and you can see if it works and in case it doesn’t, what errors it gives you.