There is a poorly iconed pull-down menu above the scroll arrow at the upper right of the message list pane. In this menu are two occurrences of the item "Who." The top one enables Recipients; the bottom one enables Senders (I have only the bottom one checked). This feature is by no means clear - I worked it out through trial and error. The first trick is to find the blasted pull-down menu. The Recipients/Senders feature is a Penelope add-on item. It's not present in the Thuderbird or even Eudora without Penelope.

By the way, I've found that Penelope has serious problems in OS X 10.6.1. With a message selected in the message pane, any keyboard input moves one or another message to the trash. If you have the trash in the message window, keyboard input will delete a selected message entirely. As a result, I have had to disable Penelope in Eudora, which rather defeats the purpose of using the Eudora version of Thunderbird. I have not reported this issue because filing bug reports for Eudora remains, as it has been for a very long time, incredibly tedious. As always with Eudora, you just have to hope they will find and fix the bug eventually - I call that the Eudora tax. Even though the program is now free, that tax remains. The developers continue to hide behind a screen of open source gobbledegook. Given that they make user feedback so difficult, I wonder why they even bother with the Eudora/Penelope project. Given the long intervals between updates, they clearly don't bother very much.