thunderbird: Update to 78.3.1

Changelog:
78.3.1
Fixes
Thunderbird crashed after updating to 78.3.0

78.3.0
Changes
OpenPGP: Improved decryption performance with large messages

OpenPGP: Do not show external key UI when disabled by preference

Account setup wizard will now open a popup when connecting to a server with a
self-signed SSL/TLS certificate

Installation of "legacy" MailExtensions now disabled

Reply-To header moved in compose window; now appears under From header

Calendar: Sidebar UI improvements

Fixes
Selecting "Cancel" on the Master Password prompt at startup incorrectly
reported corrupted OpenPGP data

OpenPGP: Creating a new key pair did not automatically select it for use

Dragging & Dropping recipient pills resulted in lost pills when an error was
present

Spellcheck suggestions were unreadable in dark theme

Calendar: Multiple password prompts opened

Linux Distributions: UI was not rendered completely when built without updater

MailExtensions: browser.folders.delete failed on IMAP folders

Various security fixes

Security fixes:
Mozilla Foundation Security Advisory 2020-44
#CVE-2020-15677: Download origin spoofing via redirect
#CVE-2020-15676: XSS when pasting attacker-controlled data into
a contenteditable element
#CVE-2020-15678: When recursing through layers while scrolling, an iterator may
have become invalid, resulting in a potential use-after-free scenario
#CVE-2020-15673: Memory safety bugs fixed in Thunderbird 78.3

(ryoon)