Thunderbird update not available

I’m using elementary OS 5.1.7 Hera and Thunderbird 68.10.0.

Since there’s a security warning on versions prior 78.6.1 I tried to update, with no success.

• AppCenter has no updates for Thunderbird.
• After apt update and apt upgrade I tried apt install --only-upgrade thunderbird, which resulted in a message saying Thunderbird was already the latest version (1:68.10.0+build1-0ubuntu0.18.04.1).

What now? Any ideas?

Thanks in advance – Stefan