diff --git a/build/leaktest.py.in b/build/leaktest.py.in
index 96bb3e48f01..afd75ea61a9 100644
--- a/build/leaktest.py.in
+++ b/build/leaktest.py.in
@@ -75,7 +75,8 @@ if __name__ == '__main__':
     browserEnv = dict(os.environ)
 
     browserEnv["NO_EM_RESTART"] = "1"
-    browserEnv["XPCOM_DEBUG_BREAK"] = "warn"
+    if not "XPCOM_DEBUG_BREAK" in browserEnv:
+        browserEnv["XPCOM_DEBUG_BREAK"] = "stack"
     if automation.UNIXISH:
         browserEnv["LD_LIBRARY_PATH"] = os.path.join(SCRIPT_DIR, DIST_BIN)
         browserEnv["MOZILLA_FIVE_HOME"] = os.path.join(SCRIPT_DIR, DIST_BIN)