Mozilla released an update for the Thunderbird 24.x branch on March 18th, 2014. Thunderbird 24.4.0 addressed these issues:

• Security fixes can be found here
• Several fixes to improve handling of BCC when replying to messages (bug 968270, Bug 969358)

Depending on update settings, users should automatically be prompted to update to Thunderbird 24.4.0 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 24.5.0 on April 29, 2014.