Mozilla has pushed out an update for Thunderbird 12.0 to address:

• Issues relating to new mail notifications and filtering on POP3 based accounts
• Occasional startup crash seen in TB 12.0
• Issue with corrupted message bodies when using movemail

Existing Thunderbird 12 users should be able to update via Help > About Thunderbird or may manually download and install Thunderbird 12.0.1 from here. A reminder, support for Thunderbird 3.1 has ended.