I am locking the voting because there is no end to this discussion and there are number of different views.
Your question was a valid one imho: "Which browser do you thing should be the official browser."
It's understandable why Opera is not the 'official' browser but that doesn't have to stop anyone from making it their personal
official browser. Firefox can be a candidate but to some people even Firefox isn't free enough: see IceWeasel
initiatives. Different views indeed but it helps to know alternatives and to know other people's views.
In the KDE Control Center under KDE Components, Component Chooser you can set a default for your system.