Previously I have been using kdocker to minimize my Thunderbird to the tray area of my Ubuntu system. With 10.04 and Thunderbird 3 kdocker no longer works.
To correct that issue I started using a new program called Minimize to Tray Plus
This addon for Thunderbird works the same as kdocker but it actually works for 10.04 and Thunderbird 3.
Here is what I did to get the addon in my Thunderbird:
1. Start Thunderbird
2. Click on the Tools Menu
3. Click on addons
4. Click Get addons
5. Enter Minimize to Tray Plus
6. Download and restart Thunderbird