--- a/testing/web-platform/tests/tools/wptrunner/wptrunner/browsers/firefox.py
+++ b/testing/web-platform/tests/tools/wptrunner/wptrunner/browsers/firefox.py
@@ -329,20 +329,21 @@ class FirefoxBrowser(Browser):
         """Write a line of output from the firefox process to the log"""
         if "GLib-GObject-CRITICAL" in line:
             return
         if line:
             data = line.decode("utf8", "replace")
             if self.stack_fixer:
                 data = self.stack_fixer(data)
             if self.lsan_handler:
-                self.lsan_handler.log(data)
-            self.logger.process_output(self.pid(),
-                                      data,
-                                      command=" ".join(self.runner.command))
+                data = self.lsan_handler.log(data)
+            if data is not None:
+                self.logger.process_output(self.pid(),
+                                           data,
+                                           command=" ".join(self.runner.command))
 
     def is_alive(self):
         if self.runner:
             return self.runner.is_running()
         return False
 
     def cleanup(self):
         self.stop()