/usr/src/linux-headers-4.4.0-89-lowlatency/firmware is in linux-headers-4.4.0-89-lowlatency 4.4.0-89.112~14.04.1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/src/linux-headers-4.4.0-89-lowlatency/../linux-headers-4.4.0-89/firmware