It’s apparently been around for awhile, but this is the first I’ve heard of it. Mozdev has developed a “minimize to tray” extension for Firefox and Thunderbird.
The extension can be easily installed via the addons menu in Thunderbird, and the icon can be manually changed to whatever you like by simply renaming an icon file placed in the Thunderbird chrome directory, the one featured in the photo is from the mozdev website offered as an alternative to the thunderbird logo, and it looks just right for Windows 7.
You can specify single or double click to maximize, and send to tray on close as well as minimize. It’s an excellent step toward integration with Windows 7, especially since you can specify tray behaviors.

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