Log Message:
thunderbird: Update to 68.4.2

Changelog:
changed
Calendar: Task and Event tree colours adjusted for the dark theme

fixed
Retrieval of S/MIME certificates from LDAP failed
Address-parsing crash on some IMAP servers when preference \ 
mail.imap.use_envelope_cmd was set
Incorrect forwarding of HTML messages caused SMTP servers to respond with a timeout
Calendar: Various parts of the calendar UI stopped working when a second \ 
Thunderbird window opened