Mozilla released an update for the Thunderbird 24.x branch on December 10th, 2013. Thunderbird 24.2.0 addressed these issues:
Security fixes can be found here
Fixed an issue where long messages with multiple signatures could end up unreadable (bug 929006)
Fixed an issue where editing account settings was not possible in some non-standard configurations of local folder set-ups (bug 921371)
Depending on update settings, users should automatically be prompted to update to Thunderbird 24.2.0 or may do so manually via Help > About Thunderbird or downloading and installing via the getthunderbird.com site. The next scheduled update will be Thunderbird 24.3.0 on January 21, 2014.