To ensure your security, always download legacy software from official sources rather than third-party mirrors which may contain malware.

Thunderbird 68.12.0 Download: Everything You Need to Know Released on , Thunderbird 68.12.0 was the final update in the 68.x release cycle before the major transition to version 78.0. While modern versions have since been released, many users still seek this specific legacy version to maintain compatibility with older extensions (add-ons) or to run on older operating systems. Where to Securely Download Thunderbird 68.12.0

At least 1GB of RAM and 200 MB of hard drive space. W12.0? | Thunderbird Support Forum