Just like Firefox, you can customize the Thunderbird email app by tweaking its internal configuration file, and tech blog Gyaan Sutra lists 8 useful edits. Set Thunderbird to top-post on replies, display threaded conversations (ala Gmail), add a default CC, and add all incoming message from: addresses to your contacts. At least some of these can be done within Thunderbird's regular interface (versus the Config Editor). See also our top Firefox config tweaks and eight killer Thunderbird extensions.