Well, I think it's a really great move.
It doesn't bring down usability. Well, it does if that's what you are use to - there's a learning curve when dealing with a new UI.
When I began programming, I released a shareware document word processor program. It was nifty for its time. I kept on adding more and more features. I emphasized menus because that's what I found the best in regards to screen real estate. When I started hearing back from users, they were confused with the software and disliked it. The much preferred the font/etc bar (you know, says font size, type, etc). Looking back on it, I realized that the whole menu emphasis made me a lazy programmer with no consideration for UI. The majority of users don't want to have to go through menus in order to go to a different screen or more submenus to alter options. A ribbon style interface updates software from its DOS stylized days. Hardware and Operating systems have changed so much in the past 25 years, why should we keep the software running on these new operating systems the same (ie. DOS days - when resolution was an issue)?
It's also a fallacy that the people at Mozilla will be ignoring other issues when designing their new UI. Why does a new UI mean that they wouldn't improve "UI responsiveness, fast drawing of loading websites and better & smoother scrolling".
You use Opera as an example of a company who values usability and simple things. Really? Opera? I mean, I like Opera - mouse gestures, built in torrent handling, etc. Even their newer Opera Mini 5 with their new UI. Howerver, Opera has added so much to their browser that to call it "simple" is perplexing... Not to be mean here - but there's a difference between efficiency and simpleness. Being simple doesn't mean it's inherently efficient.