Since bug 550487 has landed, Thunderbird shows multiple recipients on the To and Cc lines of the headers, and includes a count of how many recipients on a header line are not currently being displayed. It does not, however, dynamically adjust this set and count of recipients as the window is resized. I started writing the code to do that, but ultimately decided that I didn't have time to shake out all the bugs for 3.1. Rather than making 3.1 eat a bunch of extra risk, I'm spinning off this bug for future development. I'm attaching a work-in-progress patch. Anyone wishing to pick this up and drive it forward is encouraged to start by reading bug 550487 comment 50 and onwards to understand some of the things that need to be figured out.