But K-Meleon is not a substitute for Opera 12.
True. I wasn't suggesting it was though. I'm just saying that K-Meleon shows that you can use a native UI around Gecko (as in, using Gecko directly) where the UI actually has some features. Given that, I think it's possible to do an Opera 12-like UI, with all of Opera 12's UI features while using Gecko as a rendering engine (without using/compiling in XUL support).