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Ignore the people saying it is to do with it being an ex-business PC. Nothing to do with that.
It is your antivirus. Avira, like many other antivirus packages, installs itself as a "man in the middle" in your browser so that it can intercept your browsing traffic, ostensibly to screen it for viruses and malware. Because it has control of your browser, Firefox displays the "your browser is being managed by your organisation" message. Try uninstalling your antivirus temporarily, restart your computer and the message should be gone.
It is generally a very bad idea to allow antivirus software to hook itself into your browser. While they claim to be protecting you from web-based malware, by bypassing your browser's in built security they are simply adding more of an attack surface for malware to get into your computer. Firefox/Chrome both have very good security, as does Windows 10/11 with its inbuilt antivirus. You don't need third party antivirus software these days, it just slows down your computer.