Mozilla Thunderbird is free and open source software, built by a community of thousands from all over the world. There are a few things you should know:
Thunderbird is made available to you under the terms of the Mozilla Public License. This means you may use, copy and distribute Thunderbird to others. You are also welcome to modify the source code of Thunderbird as you want to meet your needs. The Mozilla Public License also gives you the right to distribute your modified versions.