diff --git a/test/common.py b/test/common.py
index b16d2cceeef0a..d5f6b70456d26 100644
--- a/test/common.py
+++ b/test/common.py
@@ -116,6 +116,9 @@ def configure(data_dir):
     """Chrome has no special configuration step."""
 
 
+# N.b. The earliest Firefox version that can run multithreaded browser harness
+# is Firefox 67. If you need to test earlier Firefox versions, use EMTEST_CORES=1
+# environment variable to revert to the single-threaded browser harness.
 class FirefoxConfig:
   data_dir_flag = '-profile '
   default_flags = ('-new-instance',)