/usr/share/xul-ext/adblock-plus-element-hiding-helper/chrome/locale/es-ES/overlay.dtd is in xul-ext-adblock-plus-element-hiding-helper 1.2.2-1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 | <!ENTITY helpbox.title "Selección de elementos - Teclas de acceso rápido">
|