It's a size of paper with an aspect ratio of 1:√2, and the short edge that is 21cm long. The long edge will then be 21√2 = 29.7cm. The aspect ratio has the interesting property that it can be halved and doubled while remaining constant.
This has been your ISO fact of the day.
They're doing this at the OS level, so Firefox can't protect you from that, the issue is with Windows. They could do the same to Firefox, they just don't bother.