Starting with Thunderbird version 52, Thunderbird requires Mac OS X 10.9 (Mavericks) or higher. If you are on an earlier version of Mac OS X, there are options for you to continue using a secure version of Thunderbird.
Option 1: Upgrade to a Mac OS version supported by Apple
To ensure the safest email experience, we recommend upgrading to a Mac OS version that Apple supports. Visit this Apple Support Page to learn more about upgrading.
While Thunderbird will still continue to work on unsupported versions, using Thunderbird on a supported version will provide you with the best and safest user experience.
Option 2: Download Thunderbird version 45
Thunderbird 45 will continue to work with Mac OS X 10.6, 10.7 and 10.8 but will not receive any further updates or security fixes. Thunderbird 45 can be downloaded from here. First pick the folder for your locale and then download the package.