Bug 1529394 pref 'on' incognito not_allowed r=kmag

Differential Revision: https://phabricator.services.mozilla.com/D21302

--HG--
extra : moz-landing-system : lando

modules/libpref/init/StaticPrefList.h

@@ -530,11 +530,18 @@ VARCACHE_PREF(
 // Extension prefs
 //---------------------------------------------------------------------------
 
+#ifdef ANDROID
+// Private browsing opt-in is only supported on Firefox desktop.
+# define PREF_VALUE true
+#else
+# define PREF_VALUE false
+#endif
 VARCACHE_PREF(
   "extensions.allowPrivateBrowsingByDefault",
    extensions_allowPrivateBrowsingByDefault,
-  bool, true
+  bool, PREF_VALUE
 )
+#undef PREF_VALUE
 
 //---------------------------------------------------------------------------
 // Full-screen prefs