Have you contacted that vendor to see if any changes have been made? Have you made changes to Thunderbird since the last time you received messages from that third party? If so, what were they?

Is it the "From" information that is not to your liking? Are you talking about when viewing the message itself or its listing in the Message List, or both? If the list only and the column name is "Correspondents", try adding a "From" column, which can show slightly different information.

Another setting you can try is found at <Settings | General | Message List>. It has three "Preferred address display format" settings. One of them might be more to your liking.

The actual "From' field is created by the "third-party application" and you can't change that.

Another though occurs to me - if there are a finite number of customers, you could create a Tag for each one, then create a Message Filter that would color the messages from each or move them into their own folder.