Log Message:
Update to 52.6.0

Changelog:
    Fixed Searching message bodies of messages in local folders, including
            filter and quick filter operations, not working reliably:
            Content not found in base64-encode message parts, non-ASCII text
            not found and false positives found.
    Fixed Defective messages (without at least one expected header) not shown
            in IMAP folders but shown on mobile devices
    Fixed Calendar: Unintended task deletion if numlock is enabled
    Fixed Various security fixes

Security fixes:
#CVE-2018-5095: Integer overflow in Skia library during edge builder allocation
#CVE-2018-5096: Use-after-free while editing form elements
#CVE-2018-5097: Use-after-free when source document is manipulated during XSLT
#CVE-2018-5098: Use-after-free while manipulating form input elements
#CVE-2018-5099: Use-after-free with widget listener
#CVE-2018-5102: Use-after-free in HTML media elements
#CVE-2018-5103: Use-after-free during mouse event handling
#CVE-2018-5104: Use-after-free during font face manipulation
#CVE-2018-5117: URL spoofing with right-to-left text aligned left-to-right
#CVE-2018-5089: Memory safety bugs fixed in Firefox 58, Firefox ESR 52.6,
                  and Thunderbird 52.6