The “start button” is the kde plasma logo. So this would be Linux of some sort (makes sense given the community and what OP has said) and not windows
The question is just whether OP is using steamOS that comes on the deck (and uses KDE plasma for desktop mode) or if they have installed a different distro that fits the desktop use case a bit better.
It isn’t just about ungoogling things though. Having a monoculture in the browser space means that if Google makes a push to favor ads, say by removing certain extension support from their browser engine that everyone uses, then the entire internet suffers. It is effectively a monopoly.
Mozilla tries really hard sometimes to be unappealing, but there is value in not just letting Google have full control over the internet.