I opted in to the KDE extension for Firefox for my brand new Kubuntu installation on a separate drive. How the **** did the extension manage to then also be installed in Firefox for Windows 10 on a completely separate hard drive? That the kind of **** viruses and malware do :/

Calm down and read up on extensions in Firefox: those apply to all Firefox instances run with the same account. If you want different layouts, use different accounts. Not a KDE or a Plasma issue, just normal behavior in Firefox.