Sorry for the delay in response to your query and apologize for the inconvenience caused. As per the description above, you want to add Thunderbird as the default email program in Adobe Reader DC, Is that correct?
I'm having the same issue. Thunderbird is already set as the default email app in Windows 10. For some reason, Acrobat DC sees MS Outlook as default and only gives me the option of adding a webmail service.
I tried the Acrobat is unable to connect to your email program link in your response to uzbora but it does not allow "clicking" ?