The latest supported version of Thunderbird can always be downloaded from the CTS software page.

Windows installation

1. Double-click the Thunderbird Setup 1.0.exe icon (version 1.0 is the current version, but changes occur quite frequently).
2. The Welcome to Mozilla Thunderbird window will appear. Click Next.
3. Click I Accept the Terms, then click Next.
4. In  the Setup Type window, select Standard, then click Next.
5. In the Select Components window, select Next.
6. When the installation is done, click Finish. This will finish the setup of Thunderbird and launch the application.

MacOS installation

1. Double-click the Thunderbird.dmg file. Thi should mount and automatically open the disk image.
2. Drag and copy the Thunderbird file within the disk image to your Applications folder.
3. When the copy is complete, you can drag the mounted disk image and the .dmg file to your Trash.
4. Double-click the Thunderbird icon in your Applications folder to launch the application.