Thunderbird recently released version 3 with a new feature: Content tabs. This feature is similar to browser tabs which enables you to work with many emails simultaneously!
Here is a small trick to use Google Wave in Thunderbird. To try this, go to Tools > Error Console, enter the following code and click the “Evaluate” button:
Components.classes['@mozilla.org/appshell/window-mediator;1'].getService(Components.interfaces.nsIWindowMediator).
getMostRecentWindow("mail:3pane").document.
getElementById("tabmail").openTab("contentTab", {contentPage: "https://wave.google.com/wave/?nouacheck"});

Cool! :)
[ via AddictiveTips and Joshua Cranmer ]
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Excellent tip. But to avoid having to type code in the error console, you can install the Thunder Apps extension :
https://addons.mozilla.org/en-US/thunderbird/addon/70001