/usr/share/mozilla/extensions/{3550f703-e582-4d05-9a08-453d09bdfdc6}/elemhidehelper@adblockplus.org is in xul-ext-adblock-plus-element-hiding-helper 1.2.2-1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/mozilla/extensions/{3550f703-e582-4d05-9a08-453d09bdfdc6}/../../../xul-ext/adblock-plus-element-hiding-helper