De acordo com essa edição , ela ainda não está implementada.
A extensão Compose foi abandonada.Status: NEW
Reported: 2008-08-05 16:14 PDT by Susurrus
Assigned To: Nobody; OK to take it and work on it
Importance: -- enhancement with 207 votes
With Thunderbird having a tab interface, it now makes sense to have the compose window default to opening in a new tab versus in a new window. This should probably be a default option but then also have a separate keyboard shortcut; maybe CTRL+N could be used for new window like it is now and CTRL+SHIFT+N could be used for opening in a new tab.
I really think the default should be changed to open in a new tab, but people don't really like change, so maybe give them the option now and change it in the future?
Attention: this addon is a proof of concept, it's experimental and unmaintained. Use at your own risk! We're actively looking for contributors to help with the development. Come and check out our GitHub project!
This addon allows you to write message in a new messenger tab. It uses CKEditor, and replaces the old compose window.