Where is the chrome directory?

Hi,

I just started learning XUL by reading the XUL tutorial[1]. In the "XUL Structure" chapter[2], it says:

  The packages that are provided with Mozilla are located within the chrome directory, which is in the directory where you installed Mozilla. The chrome directory is where you find all the files that describe the user interface used by the Mozilla browser, mail client, and other applications.

But I didn't find the chrome directory in my Firefox directory (/Applications/FirefoxAurora.app). I only found a chrome directory in mozilla-central[3]. Can anyone point me in the right direction? I'm using Firefox 34.0a2 on OS X 10.9.

[1] https://developer.mozilla.org/en-US/docs/XUL_Tutorial
[2] https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/XUL_Structure#Package_Organization
[3] http://dxr.mozilla.org/mozilla-central/source/chrome