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Firefox, you are probably aware of this, however: MF hasn't got the complete Tabbed Environment. Sometimes when you click links from external applications they open in a New Window. I hated when that happened. To act it like Opera, you will need Tabbrowser Preferenced (TBP).
http://www.pryan.org/mozilla/site/TheOneKEA/tabprefs/
I have heard that TBE (Tab Browser Extensions) is not recommended to be used. So everybody who are using TBE should move to TBP or something similar.