Geeks worldwide have been using IMAP mail servers to, among other things, allow them to have a consistent view of their email and their folders from various clients. Unfortunately, many useful IMAP features have had poor and spotty client support. The most notable for me has been server side tagging. There is the "!Important" flag... but come on... how much information can you really store with one flag? Is it something I need to do? Something I need to follow-up on? Something I just want to be sure I don't delete? Sure you can use Labels, but most clients store those in local databases so when you pull up your mail in the office (or at home) all of your hard work tagging those mails is now invisible to you! Enter Thunderbird 2.0 with support for IMAP Keywords in the form of tags. So long as you restrict yourself to the default 5 tags mentioned in the latest IMAP spec (or rfc, or whatever it is) these tags are stored on the server! No kidding! The trick is to rename the existing Tags from the default Thunderbird profile rather than delete those and create your own. If you do that, they become custom tags and don't get propagated, at least they didn't appear to for me. Once you rename your tags, your prefs.js file should look something like this:

user_pref("mailnews.tags.$label1.color", "#FF0000");
user_pref("mailnews.tags.$label1.tag", "Important");
user_pref("mailnews.tags.$label2.color", "#CC33CC");
user_pref("mailnews.tags.$label2.tag", "Follow-Up");
user_pref("mailnews.tags.$label3.color", "#CC9933");
user_pref("mailnews.tags.$label3.tag", "Someday");
user_pref("mailnews.tags.$label4.color", "#3333FF");
user_pref("mailnews.tags.$label4.tag", "To Do");
user_pref("mailnews.tags.$label5.color", "#6600CC");
user_pref("mailnews.tags.$label5.tag", "Waiting");

If instead you find entries that look like:

user_pref("mailnews.tags.someday.tag", "Someday");
user_pref("mailnews.tags.someday.color", "#3333FF");

Then you are using custom labels. I found myself in this boat, so I edited prefs.js and added all the $label[1-5] pairs and then did a manual search to retag all my existing mail with the new labels. At this point it's a good idea to copy your prefs.js to your other thunderbird installations so the client knows to look for those tags and render them in the same colors - lest you confuse yourself ;-)

Now... if only tbird would inline plain text attachments in replies....

References:

1. http://deflexion.com/2006/05/server-side-message-labels
2. http://www.mozilla.com/en-US/thunderbird/