Did you start Windows in safe mode? Startup crashes are often due to incompatible security/AV programs, which are bypassed in safe mode. Bitdefender is one that's caused problems recently.

Ok, I restarted Windows in safe mode and sure enough, Thunderbird worked. I immediately uninstalled Bitdefender and installed another Virus scanner. Thunderbird is working again. Thank you.

Can confirm. The problem is caused by Bitdefender's Advanced Threat Defense module.

If you disable said module or if you add Thunderbird.exe to the exceptions the crash doesn't occur.

deimios666 said

Can confirm. The problem is caused by Bitdefender's Advanced Threat Defense module. If you disable said module or if you add Thunderbird.exe to the exceptions the crash doesn't occur.

Hi deimios666.

https://forum.bitdefender.com/index.php?/topic/81641-latest-update-breaks-thunderbird/&do=findComment&comment=293676 describes a process to submit information to the vendor, information then need to diagnose the problem.

If you can give them that information it would be very helpful.

If you still crash with Bitdefender enabled, please create a new topic https://support.mozilla.org/en-US/questions/new/thunderbird/form and put Bitdefender in the title. Thanks