Thunderbird 3.1.9 fixes the following issues in Thunderbird 3.1.8:

Prevents a crash after update, that is affecting some users.
For a more detailed list of bug fixes, see the Rumbling Edge for a Thunderbird-focused list, or the complete list of changes in this version.

You may also be interested in the Thunderbird 3.1.8 release notes for a list of changes in the previous version. See the Thunderbird 3.1 release notes for a list of what is new in Thunderbird 3.1.