/usr/share/pixmaps/firefox-esr.png is in firefox-esr 52.8.1esr-1~deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/pixmaps/../firefox-esr/browser/chrome/icons/default/default48.png
/usr/share/pixmaps/../firefox-esr/browser/chrome/icons/default/default48.png