firefox

sudo dpkg-divert --rename /usr/share/pixmaps/mozilla-firefox.png
sudo dpkg-divert --rename /usr/share/pixmaps/mozilla-firefox.xpm
sudo dpkg-divert --rename /usr/lib/mozilla-firefox/icons/default.xpm
sudo dpkg-divert --rename /usr/lib/mozilla-firefox/icons/document.png
sudo dpkg-divert --rename /usr/lib/mozilla-firefox/chrome/icons/default/default.xpm
sudo cp mozilla-firefox.png /usr/share/pixmaps/mozilla-firefox.png
sudo cp mozilla-firefox.xpm /usr/share/pixmaps/mozilla-firefox.xpm
sudo cp mozilla-firefox.xpm /usr/lib/mozilla-firefox/icons/default.xpm
sudo cp mozilla-firefox-doc.png /usr/lib/mozilla-firefox/icons/document.png
sudo cp mozilla-firefox.xpm /usr/lib/mozilla-firefox/chrome/icons/default/default.xpm

thunderbird

sudo dpkg-divert --rename /usr/share/pixmaps/mozilla-thunderbird.xpm
sudo dpkg-divert --rename /usr/share/pixmaps/mozilla-thunderbird-menu.xpm
sudo dpkg-divert --rename /usr/share/pixmaps/mozilla-thunderbird-pm-menu.xpm
sudo dpkg-divert --rename /usr/lib/mozilla-thunderbird/chrome/icons/default/mozilla-thunderbird.xpm
sudo dpkg-divert --rename /usr/lib/mozilla-thunderbird/chrome/icons/default/messengerWindow16.xpm
sudo dpkg-divert --rename /usr/lib/mozilla-thunderbird/chrome/icons/default/messengerWindow.xpm
sudo dpkg-divert --rename /usr/lib/mozilla-thunderbird/chrome/icons/default/default.xpm
sudo cp mozilla-thunderbird.xpm /usr/share/pixmaps/mozilla-thunderbird.xpm
sudo cp mozilla-thunderbird.xpm /usr/share/pixmaps/mozilla-thunderbird-menu.xpm
sudo cp mozilla-thunderbird.xpm /usr/share/pixmaps/mozilla-thunderbird-pm-menu.xpm
sudo cp mozilla-thunderbird.xpm /usr/lib/mozilla-thunderbird/chrome/icons/default/mozilla-thunderbird.xpm
sudo cp mozilla-thunderbird.xpm /usr/lib/mozilla-thunderbird/chrome/icons/default/messengerWindow16.xpm
sudo cp mozilla-thunderbird.xpm /usr/lib/mozilla-thunderbird/chrome/icons/default/messengerWindow.xpm
sudo cp mozilla-thunderbird.xpm /usr/lib/mozilla-thunderbird/chrome/icons/default/default.xpm