Debian by default comes with Iceweasel which is “based on firefox”, but not quite the same as installing the original Firefox.

To install Firefox, do the following:

Add a package source to apt:

sudo emacs /etc/apt/sources.list.d/firefox.list

Now add the following line to this file:

deb debian import[/code]
Now, add the key for this source:
sudo apt-key adv --recv-key --keyserver 3EE67F3D0FF405B2

Now update your sources and install as usual:

sudo apt-get update
sudo apt-get install firefox