--- a/testing/geckodriver/README.md
+++ b/testing/geckodriver/README.md
@@ -338,16 +338,17 @@ removed once the interactability checks 
   <td><code>level</code>
   <td>string
   <td>Set the level of verbosity of geckodriver and Firefox.
    Available levels are <code>trace</code>,
    <code>debug</code>, <code>config</code>,
    <code>info</code>, <code>warn</code>,
    <code>error</code>, and <code>fatal</code>.
    If left undefined the default is <code>info</code>.
+   The value is treated case-insensitively.
  </tr>
 </table>
 
 
 `prefs` object
 --------------
 
 <table>

--- a/testing/marionette/prefs/marionette.js
+++ b/testing/marionette/prefs/marionette.js
@@ -9,18 +9,21 @@
 
 // Starts and stops the Marionette server.
 pref("marionette.enabled", false);
 
 // Delay server startup until a modal dialogue has been clicked to
 // allow time for user to set breakpoints in the Browser Toolbox.
 pref("marionette.debugging.clicktostart", false);
 
-// Marionette logging verbosity.  Allowed values are "fatal", "error",
-// "warn", "info", "config", "debug", and "trace".
+// Verbosity of Marionette logger repository.
+//
+// Available levels are, in descending order of severity,
+// "trace", "debug", "config", "info", "warn", "error", and "fatal".
+// The value is treated case-insensitively.
 pref("marionette.log.level", "info");
 
 // Port to start Marionette server on.
 pref("marionette.port", 2828);
 
 // Sets recommended automation preferences when Marionette is started.
 pref("marionette.prefs.recommended", true);