Thunderbird 3.1.3 provides improvement to the user interface and fixes several stability/security issues.
MFSA 2010-63 Information leak via XMLHttpRequest statusText
MFSA 2010-62 Copy-and-paste or drag-and-drop into designMode document allows XSS
MFSA 2010-61 UTF-7 XSS by overriding document charset using
If you do not receive an update notice when using the program, select “Check for Updates” from the Help menu.