Fonts configuration is always rather subjective. I often don't like defaults. This applies to any system. Good ones allow greater flexibility. Bad ones assume they know best and force "best defaults" without any way to improve them.
Is not about font taste, is about brokenness, you can clearly see in the screenshot that the fonts are rendered poorly, some letters are lighter than others, half of a letter is lighter than the other half, some don't look smooth, etc...
Firefox renders text using whatever text rendering settings you have set up on your computer. These differ from platform to platform, and from monitor to monitor. If you don't like sub-pixel rendering, you just turn it off and all the programs on your machine, Firefox included, stop using it.
Others have also mentioned that the subpixel order differs from monitor to monitor, so if the screenshot was taken on a monitor with BGR subpixels and your monitor has RGB subpixels, the text will look much, much, worse than if there had been no subpixel rendering at all. Or perhaps you, like me, have a CRT and subpixel rendering always looks wrong. It's hardly Firefox's fault.
Not sure about that system. I'm using Debian testing and it has very good quality defaults (visually) which I change anyway to suit some of my preferences. So the claim about that "distros ship horrible defaults" is not universally applicable.
