/usr/share/idl/thunderbird/nsITransportSecurityInfo.idl is in thunderbird-dev 1:52.8.0-1~deb8u1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 | /* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 *
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#include "nsISupports.idl"
interface nsIX509CertList;
[scriptable, uuid(216112d3-28bc-4671-b057-f98cc09ba1ea)]
interface nsITransportSecurityInfo : nsISupports {
    readonly attribute unsigned long    securityState;
    readonly attribute wstring          errorMessage;
    readonly attribute long             errorCode; // PRErrorCode
    /**
     * If certificate verification failed, this will be the peer certificate
     * chain provided in the handshake, so it can be used for error reporting.
     * If verification succeeded, this will be null.
     */
    readonly attribute nsIX509CertList failedCertChain;
};
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