--- a/testing/xpcshell/runxpcshelltests.py
+++ b/testing/xpcshell/runxpcshelltests.py
@@ -645,16 +645,19 @@ class XPCShellTestThread(Thread):
                 preloadEnvVar = 'MOZ_REPLACE_MALLOC_LIB'
                 libdmd = os.path.join(self.xrePath, 'dmd.dll')
 
             self.env['PYTHON'] = sys.executable
             self.env['BREAKPAD_SYMBOLS_PATH'] = self.symbolsPath
             self.env['DMD_PRELOAD_VAR'] = preloadEnvVar
             self.env['DMD_PRELOAD_VALUE'] = libdmd
 
+        if self.test_object.get('subprocess') == 'true':
+            self.env['PYTHON'] = sys.executable
+
         testTimeoutInterval = self.harness_timeout
         # Allow a test to request a multiple of the timeout if it is expected to take long
         if 'requesttimeoutfactor' in self.test_object:
             testTimeoutInterval *= int(self.test_object['requesttimeoutfactor'])
 
         testTimer = None
         if not self.interactive and not self.debuggerInfo and not self.jsDebuggerInfo:
             testTimer = Timer(testTimeoutInterval, lambda: self.testTimeout(proc))