/usr/include/thunderbird/gtest/ is in thunderbird-dev 1:38.6.0+build1-0ubuntu1.
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..
/usr/include/thunderbird/gtest/MozGtestFriend.h
/usr/include/thunderbird/gtest/gtest-death-test.h
/usr/include/thunderbird/gtest/gtest-message.h
/usr/include/thunderbird/gtest/gtest-param-test.h
/usr/include/thunderbird/gtest/gtest-printers.h
/usr/include/thunderbird/gtest/gtest-spi.h
/usr/include/thunderbird/gtest/gtest-test-part.h
/usr/include/thunderbird/gtest/gtest-typed-test.h
/usr/include/thunderbird/gtest/gtest.h
/usr/include/thunderbird/gtest/gtest_pred_impl.h
/usr/include/thunderbird/gtest/gtest_prod.h
/usr/include/thunderbird/gtest/internal/
/usr/include/thunderbird/gtest/internal/gtest-death-test-internal.h
/usr/include/thunderbird/gtest/internal/gtest-filepath.h
/usr/include/thunderbird/gtest/internal/gtest-internal.h
/usr/include/thunderbird/gtest/internal/gtest-linked_ptr.h
/usr/include/thunderbird/gtest/internal/gtest-param-util-generated.h
/usr/include/thunderbird/gtest/internal/gtest-param-util.h
/usr/include/thunderbird/gtest/internal/gtest-port.h
/usr/include/thunderbird/gtest/internal/gtest-string.h
/usr/include/thunderbird/gtest/internal/gtest-tuple.h
/usr/include/thunderbird/gtest/internal/gtest-type-util.h