Why does Bitdefender not allow an exception for Thunderbird to be added for
Check if below steps help:
1) Temporarily disable Bitdefender Protection: https://www.bitdefender.com/consumer/support/answer/28557/
2) Set exclusion in Bitdefender Antivirus: https://www.bitdefender.com/consumer/support/answer/13427/
3) Set exclusion in Bitdefender Advanced Threat Defense: https://www.bitdefender.com/consumer/support/answer/2393/
4) Re-enable real time protection in Bitdefender.
Regards