Mozilla pushed out an update for Thunderbird 15 late on Monday, September 10th. This release addresses these issues:

• Crashes when searching emails and forwarding emails (770262, 780908)
• Find and replace and some other actions in the compose window were not working correctly (787673)

User should be notified shortly of the update or may update via Help > About Thunderbird. The next scheduled release for Thunderbird will be on October 9th with Thunderbird 16.