Thunderbird 3.1.7 fixes the following issues in Thunderbird 3.1.6:
- Several fixes to improve performance, stability and security, see the Security Advisory.
- Several fixes to improve handling of large folder files stored locally.
- Several fixes to improve corruption in local copy of IMAP mailboxes.
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 3.1.6 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.