I've built that. Just to be clear: this is a personal project, not a mozilla project (even though I'm a mozilla employee).
Also - many people find it silly to use HTML instead of the operating system toolkit library. But it's not HTML or native. It can be both. In this case, the HTML code define the layout, and we can draw native widgets inside (look at the <input type=submit> tag in HTML, it's a native widget). For example, if you run the current build on Mac, you'll see that new tabs use Yosemite "vibrancy" effect. Native look and native performance, can, in theory, be achieved.
Also - many people find it silly to use HTML instead of the operating system toolkit library. But it's not HTML or native. It can be both. In this case, the HTML code define the layout, and we can draw native widgets inside (look at the <input type=submit> tag in HTML, it's a native widget). For example, if you run the current build on Mac, you'll see that new tabs use Yosemite "vibrancy" effect. Native look and native performance, can, in theory, be achieved.