/usr/include/thunderbird/mozilla/a11y/ is in thunderbird-dev 1:52.8.0-1~deb8u1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/thunderbird/mozilla/a11y/AccTypes.h
/usr/include/thunderbird/mozilla/a11y/Accessible.h
/usr/include/thunderbird/mozilla/a11y/AccessibleWrap.h
/usr/include/thunderbird/mozilla/a11y/DocAccessible.h
/usr/include/thunderbird/mozilla/a11y/DocAccessibleChild.h
/usr/include/thunderbird/mozilla/a11y/DocAccessibleChildBase.h
/usr/include/thunderbird/mozilla/a11y/DocAccessibleParent.h
/usr/include/thunderbird/mozilla/a11y/DocManager.h
/usr/include/thunderbird/mozilla/a11y/FocusManager.h
/usr/include/thunderbird/mozilla/a11y/HyperTextAccessible.h
/usr/include/thunderbird/mozilla/a11y/HyperTextAccessibleWrap.h
/usr/include/thunderbird/mozilla/a11y/IPCTypes.h
/usr/include/thunderbird/mozilla/a11y/Platform.h
/usr/include/thunderbird/mozilla/a11y/ProxyAccessible.h
/usr/include/thunderbird/mozilla/a11y/ProxyAccessibleBase.h
/usr/include/thunderbird/mozilla/a11y/ProxyAccessibleShared.h
/usr/include/thunderbird/mozilla/a11y/RelationType.h
/usr/include/thunderbird/mozilla/a11y/Role.h
/usr/include/thunderbird/mozilla/a11y/SelectionManager.h
/usr/include/thunderbird/mozilla/a11y/States.h