Security update for MozillaThunderbird

This update for MozillaThunderbird to version 68.4.1 fixes the following issues: Security issues fixed: - CVE-2019-17026: IonMonkey type confusion with StoreElementHole and FallibleStoreElement - CVE-2019-17016: Bypass of @namespace CSS sanitization during pasting - CVE-2019-17017: Type Confusion in XPCVariant.cpp - CVE-2019-17022: CSS sanitization does not escape HTML tags - CVE-2019-17024: multiple Memory safety bugs fixed Non-security issues fixed: - Various improvements when setting up an account for a Microsoft Exchange server. For example better detection for Office 365 accounts.