Thanks for the answer.
The problem is exactly as I said. I'm here, using this computer, you're there, so we can assume I'd know if Opera was asking me to set itself as default or not. It is not. It never asks for default. And it is always default. That's something we have to agree before continuing any further. Everytime I open firefox or chrome they prompt me saying they are not default and if I'd like to make them default. Opera never does that. And everytime I click a link to open a webpage, it's opera that runs.
Sorry for having to repeat myself, but maybe I wasn't clear enough the first time.
Back to the possible solutions, maybe it's a bug of this version I have? This is my installer's name: Opera_16.0.1196.73_Setup.exe
Is this beta or something? I use Windows 7 Ultimate 64, my processor is a core i7 4770
I have nothing against opera so far. Seems like a cool browser. But a few webpages I need to access don't work with it, so I'd like my default browser to be mozila firefox for those click-here-to-open-browser links inside programs.