I started writing a Thunderbird related reply here and deleted it, before looking to Thunderbird I suggest you verify that the font you have installed in the operating system is a unicode font. That question mark usually appears when the font does not have a character for item listed.

I think segoe ui is the default in Windows. So check that is displaying and is not damaged.

Hello there

Some e-mails are badly formed. This can happen if the program that sends the e-mail has a bug or is badly written.