This is not a Microsoft mail program. It does not execute scripts, it blocks remote images and disallows things like flash.

The result is that opening an email in 'Thunderbird does not allow anything to run or download and run. ipso facto there is no need to fluff around with looking at headers, before opening the mail. That does not remove issues with opening attachments, but as that requires a defined action from you the user. I would suggest not opening attachment unless they are expected in that email.

BTW, attachments are harmless unless opened. They are transmitted and stored in a special text encoded format called mime and until you select open or save, they really do not exist at all, they are a stream of text in the email. So when your anti virus says it found a virus or malware in your inbox, it will likely delete the entire contents to get at something that represents a very very small risk as it is in a non usable format.

At the end of all that if you are still determined, CTrl+U will open the message source, including the headers.

Thank you Matt for such a complete answer, even I can understand it! zixca