diff --git a/Doc/library/cookielib.rst b/Doc/library/cookielib.rst
index bf166f6..022ac13 100644
--- a/Doc/library/cookielib.rst
+++ b/Doc/library/cookielib.rst
@@ -324,6 +324,11 @@
 
    .. note::
 
+      Version 3 of the Firefox web browser no longer writes cookies in the
+      ``cookies.txt`` file format.
+
+   .. note::
+
       This loses information about RFC 2965 cookies, and also about newer or
       non-standard cookie-attributes such as ``port``.