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But I was really happy to know that there is an add-on for tweaking firefox this way. MinimizeToTray add-on for Firefox 3 is an add-on which would help you minimize your browser window to the tray icon. Not only Firefox, it will also help you to do this for Mozilla thunder bird as well.
But it is an experimental plugin and is kept under observation, so you need to have an account in Mozilla before you can install it.