You can add a custom mail header to your outgoing email in thunderbird by inserting lines like the following into your prefs.js (thanks to Seth Ladd).
user_pref(“mail.identity.id1.header.foaf”, “X-FOAF: http://umbc.edu/~finin/foaf.rdf”);
Note: you have to make sure that you et the right identity (e.g., id1, id2, …) in your preferences file. On my relatively new home machine I found only one, but on my older office machine I found six! I think these accumulated over the years as I moved through different versions of the Netscape family of mail clients. I took the easy way out and added the user_pref settings for all six of the identities. Where you find your prefs.js file will depend on what OS you are using, of course. I found mine in C:Documents and Settingstim fininApplication DataThunderbirdProfilesr6anf7ze.default
This seems like a nice way to enable all sorts of interesting email features, especially if we build on foaf. Spam filtering is an obvious use, about which many are thinking. It could also revive X-FACES, allowing your email notifier to show small image of the person sending you email.