Hook these into your build and check that everything still compiles.
Open <code>xre/toolkit/nsEmbedFunctions.cpp</code>. See that
#include "mozilla/test/TestThreadChild.h"
using mozilla::test::TestThreadChild;
get added to the file somewhere. Go to the function <code>XRE_InitChildProcess</code>. Add the following case to the switch.
case GeckoChildProcess_TestHarness:
mainThread = new TestThreadChild();
break;
(Yes, this sucks. Sorry)