Pete's recent attempt: https://tools.taskcluster.net/task-inspector/#gbGogdKSQQ2vsWQWWKqhow/

build-firefox-in-taskcluster.json

{
  "env": {
    "CommandPromptType": "Cross",
    "ExtensionSdkDir": "C:\\Program Files (x86)\\Microsoft SDKs\\Windows\\v8.1\\ExtensionSDKs",
    "Framework40Version": "v4.0",
    "FrameworkDir": "C:\\Windows\\Microsoft.NET\\Framework64",
    "FrameworkDIR64": "C:\\Windows\\Microsoft.NET\\Framework64",
    "FrameworkVersion": "v4.0.30319",
    "FrameworkVersion64": "v4.0.30319",
    "FSHARPINSTALLDIR": "C:\\Program Files (x86)\\Microsoft SDKs\\F#\\3.1\\Framework\\v4.0\\",
    "INCLUDE": "C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\INCLUDE;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\ATLMFC\\INCLUDE;C:\\Program Files (x86)\\Windows Kits\\8.1\\include\\shared;C:\\Program Files (x86)\\Windows Kits\\8.1\\include\\um;C:\\Program Files (x86)\\Windows Kits\\8.1\\include\\winrt;",
    "LIB": "C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\LIB;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\ATLMFC\\LIB;C:\\Program Files (x86)\\Windows Kits\\8.1\\lib\\winv6.3\\um\\x86;",
    "LIBPATH": "C:\\Windows\\Microsoft.NET\\Framework64\\v4.0.30319;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\LIB;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\ATLMFC\\LIB;C:\\Program Files (x86)\\Windows Kits\\8.1\\References\\CommonConfiguration\\Neutral;C:\\Program Files (x86)\\Microsoft SDKs\\Windows\\v8.1\\ExtensionSDKs\\Microsoft.VCLibs\\12.0\\References\\CommonConfiguration\\neutral;",
    "MOZILLABUILD": "C:\\mozilla-build\\",
    "MOZ_MSVCBITS": "32",
    "MOZ_MSVCVERSION": "12",
    "MOZ_MSVCYEAR": "2013",
    "MOZ_TOOLS": "C:\\mozilla-build\\moztools-x64",
    "MSVCKEY": "HKLM\\SOFTWARE\\Wow6432Node\\Microsoft\\VisualStudio\\12.0\\Setup\\VC",
    "Path": "C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\Common7\\IDE\\CommonExtensions\\Microsoft\\TestWindow;C:\\Program Files (x86)\\MSBuild\\12.0\\bin\\amd64;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\BIN\\amd64_x86;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\BIN\\amd64;C:\\Windows\\Microsoft.NET\\Framework64\\v4.0.30319;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\VCPackages;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\Common7\\IDE;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\Common7\\Tools;C:\\Program Files (x86)\\HTML Help Workshop;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\Team Tools\\Performance Tools\\x64;C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\Team Tools\\Performance Tools;C:\\Program Files (x86)\\Windows Kits\\8.1\\bin\\x64;C:\\Program Files (x86)\\Windows Kits\\8.1\\bin\\x86;C:\\Program Files (x86)\\Microsoft SDKs\\Windows\\v8.1A\\bin\\NETFX 4.5.1 Tools\\x64\\;C:\\Windows\\System32;C:\\Windows;C:\\Windows\\System32\\Wbem;C:\\mozilla-build\\moztools-x64\\bin;C:\\mozilla-build\\7zip;C:\\mozilla-build\\info-zip;C:\\mozilla-build\\kdiff3;C:\\mozilla-build\\mozmake;C:\\mozilla-build\\nsis-3.0b1;C:\\mozilla-build\\nsis-2.46u;C:\\mozilla-build\\python;C:\\mozilla-build\\python\\Scripts;C:\\mozilla-build\\upx391w;C:\\mozilla-build\\wget;C:\\mozilla-build\\yasm",
    "Platform": "X86",
    "PreferredToolArchitecture": "x64",
    "SDKDIR": "C:\\Program Files (x86)\\Windows Kits\\8.1\\",
    "SDKMINORVER": "1",
    "SDKPRODUCTKEY": "HKLM\\SOFTWARE\\Microsoft\\Windows Kits\\Installed Products",
    "SDKROOTKEY": "HKLM\\SOFTWARE\\Microsoft\\Windows Kits\\Installed Roots",
    "SDKVER": "8",
    "TOOLCHAIN": "64-bit cross-compile",
    "VCDIR": "C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\",
    "VCINSTALLDIR": "C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\VC\\",
    "VisualStudioVersion": "12.0",
    "VSINSTALLDIR": "C:\\Program Files (x86)\\Microsoft Visual Studio 12.0\\",
    "WIN64": "1",
    "WIN81SDKKEY": "{5247E16E-BCF8-95AB-1653-B3F8FBF8B3F1}",
    "WINCURVERKEY": "HKLM\\SOFTWARE\\Microsoft\\Windows\\CurrentVersion",
    "WindowsSdkDir": "C:\\Program Files (x86)\\Windows Kits\\8.1\\",
    "WindowsSDK_ExecutablePath_x64": "C:\\Program Files (x86)\\Microsoft SDKs\\Windows\\v8.1A\\bin\\NETFX 4.5.1 Tools\\x64\\",
    "WindowsSDK_ExecutablePath_x86": "C:\\Program Files (x86)\\Microsoft SDKs\\Windows\\v8.1A\\bin\\NETFX 4.5.1 Tools\\"
  },
  "command": [
    "set",
    "hg clone https://hg.mozilla.org/mozilla-central",
    "cd mozilla-central",
    "rem mach build",
    "rem python -u testing\\mozharness\\scripts\\fx_desktop_build.py --config-file C:\\releng_base_windows_64_builds.py --branch mozilla-central --config balrog/production.py --skip-buildbot-action --build-pool production build",
    "python C:\\mozilla-source\\mozilla-central\\mach build"
  ],
  "artifacts": [
    {
      "type": "file",
      "path": "mozilla-central\\obj-i686-pc-mingw32\\dist\\bin\\firefox.exe",
      "expires": "2015-08-27T16:13:49.513Z"
    }
  ],
  "maxRunTime": 7200
}