Thunderbird has been released in the fresh version 91.8.1. You can initiate the update in the program by switching to “About Thunderbird”.
The active version is displayed there and, if necessary, the update is downloaded. As the minimal version jump already tells you, the update primarily contains minor bug fixes.
Thunderbird 91.8.1 is available from Windows 7, macOS 10.12, or Linux GTK+ 3.14, or of course the newer versions. You can find the release notes directly here.
For example, a bug has been fixed that led to duplicate notifications of new e-mails being presented to you when certain filters were created on the server-side.