This Thunderbird release adds Ubuntu One support to Filelink to complement the existing Filelink provider, YouSendIt. This will allow sending of large attachments with Ubuntu One in addition to YouSendIt.
You will see Ubuntu One when adding a new outgoing provider for attachments:
Click the menu button
and choose Options.Preferences.
click on the Attachments tab and then click on the Outgoing sub-tab and then click Add
Instant Messaging & Chat using Facebook, Twitter, GTalk, IRC and XMPP
Thunderbird 15 now supports chat: you now can enjoy real time conversation with your contacts, right from Thunderbird alongside your email, RSS and NNTP messages. Additionally, Thunderbird makes it easy to search through both past instant messaging conversations and emails.