More of them are being sold every day, and the pace is only set to increase as it's pretty much inevitable that Apple will release more high-resolution devices as time goes on. Might as well get it over with now before Firefox gets labeled as "the ugly one on high-res devices".

Are we ever going to see a high resolution Windows implementation?

Windows 8 will support multiple DPI values, at least for Metro.

This is of course true. But it will ensure every Retina MBP owner is not using Firefox.

This is a priority. The numbers are small today but they're growing and we can expect more HiDPI machines from Apple in the not too far off future.