--- a/testing/tps/tps/cli.py
+++ b/testing/tps/tps/cli.py
@@ -101,16 +101,20 @@ def main():
                                     'services', 'sync', 'tps', 'extensions')
     else:
         if sys.platform == 'win32':
             # replace msys-style paths with proper Windows paths
             m = re.match('^\/\w\/', extensionDir)
             if m:
                 extensionDir = '%s:/%s' % (m.group(0)[1:2], extensionDir[3:])
                 extensionDir = extensionDir.replace('/', '\\')
+    if sys.platform == 'darwin':
+        # Needed to avoid tab crashes on mac due to level 3 sandboxing
+        sourceRoot = os.path.join(extensionDir, '..', '..', '..', '..')
+        os.environ['MOZ_DEVELOPER_REPO_DIR'] = os.path.abspath(sourceRoot)
 
     TPS = TPSTestRunner(extensionDir,
                         binary=options.binary,
                         config=config,
                         debug=options.debug,
                         ignore_unused_engines=options.ignore_unused_engines,
                         logfile=options.logfile,
                         mobile=options.mobile,