user@host:~$ sudo apt-get update
[sudo] password for user:
Hit http://deb.whonix.org jessie InRelease
Hit http://security.debian.org jessie/updates InRelease
Ign http://ftp.us.debian.org jessie InRelease
Hit http://deb.whonix.org jessie/main i386 Packages
Hit http://security.debian.org jessie/updates/main i386 Packages
Hit http://ftp.us.debian.org jessie Release.gpg
Hit http://security.debian.org jessie/updates/contrib i386 Packages
Hit http://ftp.us.debian.org jessie Release
Hit http://security.debian.org jessie/updates/non-free i386 Packages
Hit http://ftp.us.debian.org jessie/main i386 Packages
Hit http://security.debian.org jessie/updates/contrib Translation-en
Hit http://ftp.us.debian.org jessie/contrib i386 Packages
Hit http://security.debian.org jessie/updates/main Translation-en
Hit http://ftp.us.debian.org jessie/non-free i386 Packages
Hit http://security.debian.org jessie/updates/non-free Translation-en
Hit http://ftp.us.debian.org jessie/contrib Translation-en
Hit http://ftp.us.debian.org jessie/main Translation-en
Hit http://ftp.us.debian.org jessie/non-free Translation-en
Ign http://deb.whonix.org jessie/main Translation-en_US
Ign http://deb.whonix.org jessie/main Translation-en
Reading package lists... Done
user@host:~$ sudo apt-get install firefox
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package firefox is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'firefox' has no installation candidate
user@host:~$ ^C
user@host:~$ su
Password:
root@host:/home/user# apt-get install firefox
Reading package lists... Done
Building dependency tree
Reading state information... Done
Package firefox is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source

E: Package 'firefox' has no installation candidate