Before you install Thunderbird, check that your computer meets the System Requirements.
Open the Thunderbird download page.
The page will automatically detect the platform and language on your computer and recommend the best edition(s) of Thunderbird for you. If you want to download Thunderbird in a language other than the one suggested, click on "Other Systems & Languages" for the list of available editions. Click on the OS X installation of your choice to continue.
Once the download is completed, the disk image may open by itself and mount a new volume which contains the Thunderbird application. If you do not see the new volume, double-click the Thunderbird dmg icon to open it. A Finder window appears, containing the Thunderbird application. Drag the Thunderbird icon to the Applications folder.
At this point you can eject the disk image by selecting it in a Finder window and pressing the command+E keys or by using the Finder's File menu, and selecting Eject.
Thunderbird is now ready for use. Open the Applications folder and double-click on the Thunderbird icon to start it. You may get a security warning that Thunderbird has been downloaded from the Internet. Because you downloaded Thunderbird from the official site, you can click Open to continue.
The first time you start Thunderbird you will be alerted that it is not your default email application. (The default email application is the program that opens, for example, when you click a link on a web page to an email address.) If you want Thunderbird to be the default email application, click Yes to set it as your default mailer. If not (for example if you are just trying out Thunderbird) click No.