--- a/cubicweb/devtools/qunit.py	Tue Dec 17 17:54:09 2019 +0100
+++ b/cubicweb/devtools/qunit.py	Tue Dec 17 18:04:02 2019 +0100
@@ -95,6 +95,8 @@
             assert self._process.returncode is None,  self._process.returncode
             self._process.terminate()
             self._process.wait()
+            assert self._process.returncode == 0, "Error: firefox return code is %s, see %s" %\
+                                                  (self._process.returncode, self.log_file)
             self._process = None
 
     def __del__(self):