Is there an exploit in Thunderbird? I have not heard about it. Perhaps you could offer some more information from bitdefender on exactly what exploit it thinks it is preventing.

One thing I can be sure about. Reinstalling is not going to stop bitdefender, even if it is wrong, from blocking Thunderbird at this point. So more details are needed.

Matt, thanks for replying. Unfortunately the screenshot below is all that Bitdefender is giving me.

Suggest you disable the Advanced Threat Defense option or add TB as an exception:

https://support.mozilla.org/en-US/questions/1275787