This new Windows 11 laptop I mentioned runs Windows 11 Home in S - 'secure' - mode. This turned out to mean that you can only install programs from the Windows Store. And you can't install Firefox - or probably Chrome: you have to use the Microsoft Edge browser.

It took me an hour to discover this and to switch Windows out of S-mode - it's deliberately confusing - you have to install something from the Store to remove the limitations.

Anyway, I came accross this FAQ my research:

Do I need Antivirus Software while in S mode?

Yes, we recommend all Windows devices use antivirus software. Currently, the only antivirus software known to be compatible with Windows 11 in S mode is the version that comes with it: Windows Defender Security Center. Windows Defender Security Center delivers a robust suite of security features that help keep you safe for the supported lifetime of your Windows 11 device.