It's pixel-doubled like every other non-native-Retina app on your Mac. Why is it Firefox's problem if Apple didn't bother to go to the trouble of making non-Retina applications readable on a Retina display? The fact that they don't even bother to do filtering when pixel doubling is probably a big contributor to readability issues (note that Vista and W7 do filtering when doing DPI adjustment, likely for this exact reason), not to mention the fact that it appears to be doing subpixel antialiasing when there are no subpixels to address (WTF, Apple?)