/usr/share/xul-ext/lightbeam/data/font-awesome.css 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/../../../fonts-font-awesome/css/font-awesome.css