thunderbird: Update to 78.7.0

Changelog:
What's New
Extension API: Compose API now supports editing messages and templates as new
messages

Extension API: composeHtml is now exposed in MailIdentity

Extension API: windows.update and windows.create now support titlePreface

Extension API: new Accounts API functions: accounts.getDefault() and
accounts.getDefaultIdentity(accountId)

Changes
Extension API: body and plainTextBody are now used as compose mode selectors in
setComposeDetails and begin* functions in Compose API

Theme: removed the double border around the task description field on the Tasks
tab

Fixes
Account Manager: When deleting the last remaining account, the default account
was not getting cleared and still pointed to the no-longer-existing account

OpenPGP: Verification of an inline signed message would fail if it contained
leading whitespace

OpenPGP: Various other minor bug and stability fixes

Mail Window: Quickfilter bar buttons disappear when hovered on Windows 10 High
Contrast Black theme

Theme: folder properties dialog contained black text on a black background in
dark mode

Theme: recipient pills in compose window were not visible in high contrast dark
theme on Windows 10

Extension API: browserAction buttons were not restored after restart if they
were moved outside the default toolbar

Extension API: browser.compose.beginNew could not override identity plaintext
setting

Extension API: browser.compose.beginForward was ignoring ComposeDetails

Extension API: browser.compose.setComposeDetails did not properly handle
Windows-style line endings

Various security fixes

Security fixes:
#CVE-2021-23953: Cross-origin information leakage via redirected PDF requests
#CVE-2021-23954: Type confusion when using logical assignment operators in
JavaScript switch statements
#CVE-2020-15685: IMAP Response Injection when using STARTTLS
#CVE-2020-26976: HTTPS pages could have been intercepted by a registered
service worker when they should not have been
#CVE-2021-23960: Use-after-poison for incorrectly redeclared JavaScript
variables during GC
#CVE-2021-23964: Memory safety bugs fixed in Thunderbird 78.7

(ryoon)