/usr/include/thunderbird/mtransport/ is in thunderbird-dev 1:52.8.0-1~deb8u1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/thunderbird/mtransport/dtlsidentity.h
/usr/include/thunderbird/mtransport/m_cpp_utils.h
/usr/include/thunderbird/mtransport/nricectx.h
/usr/include/thunderbird/mtransport/nricemediastream.h
/usr/include/thunderbird/mtransport/nriceresolverfake.h
/usr/include/thunderbird/mtransport/rlogconnector.h
/usr/include/thunderbird/mtransport/runnable_utils.h
/usr/include/thunderbird/mtransport/sigslot.h
/usr/include/thunderbird/mtransport/simpletokenbucket.h
/usr/include/thunderbird/mtransport/stun_socket_filter.h
/usr/include/thunderbird/mtransport/transportflow.h
/usr/include/thunderbird/mtransport/transportlayer.h
/usr/include/thunderbird/mtransport/transportlayerdtls.h
/usr/include/thunderbird/mtransport/transportlayerice.h
/usr/include/thunderbird/mtransport/transportlayerlog.h
/usr/include/thunderbird/mtransport/transportlayerloopback.h
/usr/include/thunderbird/mtransport/transportlayerprsock.h