/usr/share/pixmaps/thunderbird.png is in thunderbird 1:52.8.0-1~deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/pixmaps/../thunderbird/chrome/icons/default/default48.png
/usr/share/pixmaps/../thunderbird/chrome/icons/default/default48.png