/usr/lib/thunderbird-devel/idl is in thunderbird-dev 1:52.8.0-1~deb8u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/thunderbird-devel/../../share/idl/thunderbird
/usr/lib/thunderbird-devel/../../share/idl/thunderbird