This says nowhere that your certificate is invalid. What it says is that the signature of the message is not valid. I.e. the message has been modified in some way between sending and receiving. A possible explanation could be that your email provider injected some text into the message body (an ad, header, footer, whatever).

I originally thought it was our email server, however we host our own email server using IredMAIL, and have been tailing the logs and have found no such activity: please note this works with Outlook (We left outlook because of IMAP crashes), and other email clients we are experiencing this in Thunderbird only, which we like all the functionality over the other clients. I have read in past posts that older versions of TB would do this, something about how it handled on the client end.

I think this maybe a bug!

Thoughts?

If you think it's a bug feel free to raise one in Bugzilla. https://bugzilla.mozilla.org/