I've tested loads of different themes out, but not found any that look as good as the default one. Most clutter things up more or don't quite look right. I've not looked in some time though so some better ones might be available now.
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I always used Blue Ice with 2.0 version. It's very decent but looks very good https://addons.mozilla.org/en-US/firefox/addon/3085
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That's a nice looking theme. Shame it isn't for version 3.
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