How to list installed packages that depend on another installed package in Debian?

To find the reverse dependencies you can install the apt-rdepends package:

sudo apt-get install apt-rdepends

You can then search for reverse dependencies using:

$ apt-rdepends --state-follow=Installed --state-show=Installed -r firefox
Reading package lists... Done
Building dependency tree       
Reading state information... Done
firefox
  Reverse Depends: firefox-globalmenu (= 20.0+build1-0ubuntu2)
  Reverse Depends: firefox-gnome-support (20.0+build1-0ubuntu2)
  Reverse Depends: lightdm-remote-session-uccsconfigure (1.1-0ubuntu2)
  Reverse Depends: xul-ext-unity (>= 2.4.7bzr13.04.15-0ubuntu1)
  Reverse Depends: xul-ext-webaccounts (>= 0.4.5-0ubuntu4)
firefox-globalmenu
firefox-gnome-support
lightdm-remote-session-uccsconfigure
xul-ext-unity
xul-ext-webaccounts