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And the Mozilla mockup goes back to December. http://design-noir.de/log/2009/12/opera-firefox-ui/#comment-14680
What matters more than "who killed who" is consensus. The more apps that adopt the idea, the better.
The Mozilla mockup did not appear until two days after Opera 10.5 was leaked. They may have been presented at a Mozilla meeting, but I seriously doubt that people from Opera are attending internal Mozilla meetings!
Never mind the fact that the button is already in Opera 10.0, only disabled by default.