/usr/share/mozilla/extensions/{aa3c5121-dab2-40e2-81ca-7ea25febc110}/viewer@pdf.js is in xul-ext-pdf.js 1.5.188+dfsg-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/mozilla/extensions/{aa3c5121-dab2-40e2-81ca-7ea25febc110}/../../../xul-ext/pdf.js