"The bird will have the Lightning calendar offered as a
default installation option in its final version - and with it
introduce you to tabs. Yes, tabs as we know it from Firefox - now
in the upcoming new Mozilla product for email, messaging,
calendaring and whatnot.
"It is specially exciting to see the advances and the arrival of
real results after Mozilla renewed its commitment for this product
with the founding of Mozilla Messaging. Many of the suggestions
made by a small group of real Thunderbird lovers including myself,
as a response to the famous "Call for action" by Mitchell Baker are
actually being realized now. True, it's not all of what I could
wish for, but it was the understanding that the next big release
will not have all the whistle and bells. Instead much needed
reorganization, code cleanup and bug fixing had priority before new
features could be added."