/usr/share/xul-ext/lightbeam/resources/lightbeam/data/font/OpenSans-Light.ttf is in xul-ext-lightbeam 1.2.1+dfsg-1ubuntu2.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/xul-ext/lightbeam/resources/lightbeam/data/font/../../../../../../texlive/texmf-dist/fonts/truetype/public/opensans/OpenSans-Light.ttf