/usr/share/xul-ext/lightbeam/data/font/OpenSans-Bold.ttf is in xul-ext-lightbeam 1.3.1+dfsg-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/xul-ext/lightbeam/data/font/../../../../texlive/texmf-dist/fonts/truetype/public/opensans/OpenSans-Bold.ttf