If anything it's the other way around. Mozilla works out in the open and the designs of the new UI had been released long before Opera released its software, as Alex Limi points out: http://limi.net/articles/firefox-4/. That said, what's wrong with taking the good ideas from other products? It's a net win for the users and the web.

I was surprised also that the UI is so similar to Opera. I always wanted to go from Opera to Firefox (because Opera's lack of Adblock and some problems with google apps like calendar, gmail), but it was too slow and ugly. Now I give it a good test.

Wow, that's strikingly similar, I had no idea (I don't use Opera).