So you're looking for a really easy way to integrate Thunderbird in the Ubuntu Messaging Menu? You can try a package I've recently came across (called "xul-ext-indicator") that is available in a PPA, which adds Thunderbird to the Messaging Menu and provides quick access to your contacts, compose a new mail and unread emails. Using this package, you'll also get NotifyOSD notifications when new mail arrives.
To add the PPA and install xul-ext-indicator in Ubuntu 10.10 Maverick Meerkat, use the following commands:
sudo add-apt-repository ppa:ruben-verweij/thunderbird-indicator sudo apt-get update sudo apt-get install xul-ext-indicator
For NotifyOSD notifications, also install the following package:
sudo apt-get install libnotify-bin
Note: I've tested it with Thunderbird 3.1.x and it worked great on all 3 computers I've used (as opposed to some other solutions for adding Thunderbird to the Messaging Menu I've tried out).