Mozilla’s main bundle that is attached to a monetization scheme is the Firefox browser, which is monetized through people’s use of a particular search provider when connected through the bundle. Thunderbird (and its related Mozilla Messaging cost center) do not currently contribute anything directly to that bundle and its monetization scheme.
The obvious solution is that the underlying communications code that drives Thunderbird needs to be repackaged as a Firefox extension.
Efforts are already being made to reduce the barriers that keep the mailnews code base separate from Firefox. I don’t think it would be a huge leap to figure out how to merge Thunderbird’s user interface and tab system into FireFox’s.
If you’ve got suggestion drop them here; Lessons from Google: Thunderbird as a Firefox extension!