RE: Bye bye Firefox !!
That an the lack of any really helpful responses here.
Your lack of cooperation has a lot do with the result here: You declared that Firefox is trash and it "absolutely" damaged your Windows installation, which is simply not possible if you obtained a clean copy of the installer from an authorized source. So you're really declaring yourself to be deeply computer-illiterate.
Then you declared that something is wrong with Opera because it is slow — which is also virtually impossible. If Opera is slow all the time, the problem is with some other software or your internet connection. And this is only a forum for Opera browser support. No one here has an obligation to teach you how to use a PC, and help you fix Windows (or your internet service.)
Many of us dont really want to spend our life fixing Bill Gates software design mistakes for free so he can make billions from Windows. If you are having that much trouble with Windows and you cannot afford to pay for tech support, my advice is to stop using Windows because it requires a relatively high level of administrative intervention compared to Mac OS or Ubuntu running on a supported hardware platform.
Its possible that your system is infected with malware and even if not, the kind of support you are going to need here will require a technician to take control of your computer to use various system tools. I might even be willing to do that for free, purely out of curiosity about how you damaged your installation -- but you wont attract people willing to help when you only go around whining and blaming things you know nothing about, and refuse to state a question or answer one either!