diff -urN firefox-38.0.5.orig/python/mozbuild/mozbuild/frontend/emitter.py firefox-38.0.5/python/mozbuild/mozbuild/frontend/emitter.py
--- firefox-38.0.5.orig/python/mozbuild/mozbuild/frontend/emitter.py	2015-05-25 22:28:56.000000000 +0100
+++ firefox-38.0.5/python/mozbuild/mozbuild/frontend/emitter.py	2015-07-08 23:05:35.409977000 +0100
@@ -3,6 +3,7 @@
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 from __future__ import unicode_literals
+from __future__ import print_function
 
 import itertools
 import json
@@ -392,6 +393,8 @@
 
         for symbol in ('SOURCES', 'HOST_SOURCES', 'UNIFIED_SOURCES'):
             for src in (context[symbol] or []):
+                print('sys.argv is %s' % sys.argv, file=sys.stderr)
+                print('checking if %s exists' % (mozpath.join(context.srcdir, src)), file=sys.stderr)
                 if not os.path.exists(mozpath.join(context.srcdir, src)):
                     raise SandboxValidationError('File listed in %s does not '
                         'exist: \'%s\'' % (symbol, src), context)