Good news for the Firefox users on OS X: Firefox 18 will support retina display for smoother and un-pixelated view. This feature will also appear in Mozilla Thunderbird.
Retina display uses high pixel density, due to which human eyes fail to distinguish individual pixels. This results in to sharper display. Moreover, the Retina display uses IPS (in-plane switching) technology to achieve a wider viewing angle than on typical LCDs. Apple describes the HiDPI display, “High-resolution displays provide a rich visual experience, allowing users to see sharper text and more details in photos than on standard-resolution displays. […] When you run a high-resolution-savvy app on a high-resolution device, the text, vector drawing, and UI controls are sharp. This is due to the increased pixel density—pixels are smaller and there are more of them per unit area.”.
The following screenshot compares standard display screen and HiDPI screen, which clearly shows the sharper and smoother rendering of graphics on HiDPI screens: