Actually, none of this is exactly true. The earlier, 16 bit consumer versions of Windows - Windows 3, Win 95, Win 98, Win ME - were based on DOS. But all 32 bit and 64 bit versions of Windows - NT, 2000, XP, Vista, and Windows 7 - are based on a modern multi-tasking kernel that is not inherently any more or less secure than various Unix kernels.
I was keeping my reply deliberately simple; I'm an old UNIX hand myself.
Although I have no experience with more recent versions of Windows, I'd like to see some evidence of the claim the Windows' security is on par with UNIX's. For example, does the Wnndows file system support the concept of different users? That is, does each file on the system have associated with it an ID that is unique to each user, and are the permissions settable such that unprivileged users can't read/write other users' files? AFAIK, neither FAT nor NTFS do.
Security experts all seem to rank Firefox as the most secure browser, FWIW. If you've got Macs in the house frequently used by less cautious or computer-savvy users, switching to Firefox as the primary browser is probably not a bad idea.
I don't doubt that; I've been using Firefox (and prior to that, Mozilla) for many years.