Yep, like I mentioned (only half-joking); I really wish a web browser was useful offline . Firefox is probably the most trustworthy of the scummy bunch but they are still very clearly eroding privacy with all their "services".Which means that a web browser on the "Isolated PC" can download and execute malware.
However my main concern when running Windows is keeping the operating system as a whole offline. I.e to interrupt telemetry, consumer updates and any other issues. The start menu or lock screen has absolutely no business going online to update the visual adverts it displays.
I would always recommend running browsers in a VM (or Jail if Windows was perhaps more useful), however then you might as well simply not choose to run Windows in the first place!