One of the most recent additions to Thunderbird, aside from the built-in chat, is the ability to share large files via email with the help of a couple of cloud storage services.
For now, Thunderbird 15, the latest stable release officially supports Ubuntu One and YouSendIt.
you can add another service to the list, one of the most popular cloud storage services around, Dropbox.
The Dropbox for Filelink
add-on does exactly what you'd expect it to do, it enables you to add a file to your Dropbox account and then share the link with whoever you're emailing.
The add-on is straightforward enough to use, just add an attachment like you normally would and, if the file is too large, Thunderbird will offer to have it uploaded to a file sharing service.
You can install the add-on by going to Tools > Add-ons and search for "Dropbox for Filelink."