--- a/testing/marionette/components/marionette.js
+++ b/testing/marionette/components/marionette.js
@@ -20,25 +20,37 @@ const PREF_PORT = "marionette.port";
 const PREF_PORT_FALLBACK = "marionette.defaultPrefs.port";
 const PREF_LOG_LEVEL = "marionette.log.level";
 const PREF_LOG_LEVEL_FALLBACK = "marionette.logging";
 const PREF_FORCE_LOCAL = "marionette.forcelocal";
 const PREF_FORCE_LOCAL_FALLBACK = "marionette.force-local";
 
 const DEFAULT_PORT = 2828;
 const DEFAULT_LOG_LEVEL = "info";
-const LOG_LEVELS = new Map([
-  ["fatal", Log.Level.Fatal],
-  ["error", Log.Level.Error],
-  ["warn", Log.Level.Warn],
-  ["info", Log.Level.Info],
-  ["config", Log.Level.Config],
-  ["debug", Log.Level.Debug],
-  ["trace", Log.Level.Trace],
-]);
+const LOG_LEVELS = new class extends Map {
+  constructor () {
+    super([
+      ["fatal", Log.Level.Fatal],
+      ["error", Log.Level.Error],
+      ["warn", Log.Level.Warn],
+      ["info", Log.Level.Info],
+      ["config", Log.Level.Config],
+      ["debug", Log.Level.Debug],
+      ["trace", Log.Level.Trace],
+    ]);
+  }
+
+  get (level) {
+    let s = new String(level).toLowerCase();
+    if (!this.has(s)) {
+      return DEFAULT_LOG_LEVEL;
+    }
+    return super.get(s);
+  }
+};
 
 // Besides starting based on existing prefs in a profile and a command
 // line flag, we also support inheriting prefs out of an env var, and to
 // start Marionette that way.
 //
 // This allows marionette prefs to persist when we do a restart into
 // a different profile in order to test things like Firefox refresh.
 // The environment variable itself, if present, is interpreted as a
@@ -48,55 +60,42 @@ const LOG_LEVELS = new Map([
 // pref being set to true, thus triggering marionette being enabled for
 // that startup.
 const ENV_PREF_VAR = "MOZ_MARIONETTE_PREF_STATE_ACROSS_RESTARTS";
 
 const ServerSocket = CC("@mozilla.org/network/server-socket;1",
     "nsIServerSocket",
     "initSpecialConnection");
 
+// Get preference value of |preferred|, falling back to |fallback|
+// if |preferred| is not user-modified and |fallback| exists.
+function getPref (preferred, fallback) {
+  if (!Preferences.isSet(preferred) && Preferences.has(fallback)) {
+    return Preferences.get(fallback, Preferences.get(preferred));
+  }
+  return Preferences.get(preferred);
+}
+
 // Marionette preferences recently changed names.  This is an abstraction
 // that first looks for the new name, but falls back to using the old name
 // if the new does not exist.
 //
 // This shim can be removed when Firefox 55 ships.
 const prefs = {
   get port () {
-    let fallback = Preferences.get(PREF_PORT_FALLBACK, DEFAULT_PORT);
-    return Preferences.get(PREF_PORT, fallback);
+    return getPref(PREF_PORT, PREF_PORT_FALLBACK);
   },
 
   get logLevel () {
-    let level = DEFAULT_LOG_LEVEL;
-    let fallback = Preferences.get(PREF_LOG_LEVEL_FALLBACK, level);
-    let p = Preferences.get(PREF_LOG_LEVEL, fallback);
-
-    switch (typeof p) {
-      // Gecko >= 46
-      case "string":
-        let s = p.toLowerCase();
-        if (LOG_LEVELS.has(s)) {
-          level = LOG_LEVELS.get(s);
-        }
-        break;
-
-      // Gecko <= 45
-      case "boolean":
-        if (p) {
-          level = Log.Level.Trace;
-        }
-        break;
-    }
-
-    return level;
+    let s = getPref(PREF_LOG_LEVEL, PREF_LOG_LEVEL_FALLBACK);
+    return LOG_LEVELS.get(s);
   },
 
   get forceLocal () {
-    let fallback = Preferences.get(PREF_FORCE_LOCAL_FALLBACK, true);
-    return Preferences.get(PREF_FORCE_LOCAL, fallback);
+    return getPref(PREF_FORCE_LOCAL, PREF_FORCE_LOCAL_FALLBACK);
   },
 
   readFromEnvironment (key) {
     const env = Cc["@mozilla.org/process/environment;1"]
         .getService(Ci.nsIEnvironment);
 
     if (env.exists(key)) {
       let prefs;