From MozillaZine Knowledge Base

Thunderbird 3.0 uses mailnews.customHeaders. The string format has also changed. For example my copy of that setting is set to X-Spam-hits: Body: Reply-To. We should also switch from using user.js to the config editor in the examples once 3.0 is formally released. Tanstaafl 23:42, 6 December 2009 (UTC)