Mozilla Messaging has release the second update for Thunderbird 3.1 based on the Gecko 1.9.2 platform. For more information and to download please see the Thunderbird 3.1.2 Release Notes.

Note: There is not presently an updated for Thunderbird 3.0 at this time. Thunderbird 3.0.7 will be released with Thunderbird 3.1.3 in early September.

Source: Mozilla Developer’s News