--- a/toolkit/components/osfile/osfile_shared_allthreads.jsm
+++ b/toolkit/components/osfile/osfile_shared_allthreads.jsm
@@ -98,27 +98,41 @@
          for (let i = 0; i < arguments.length; ++i) {
            text += (" " + arguments[i]);
          }
          dump(text + "\n");
        };
      }
 
      /**
-      * Apply JSON.stringify to an argument of type object.
+      * Returns |arg.toString()| if available, otherwise
+      * applies JSON.stringify.
       * Return itself otherwise.
       *
       * @param arg An argument to be stringified if possible.
       */
      let stringifyArg = function stringifyArg(arg) {
        if (typeof arg === "string") {
          return arg;
        }
        if (arg && typeof arg === "object") {
-         return JSON.stringify(arg);
+         let argToString = arg.toString();
+
+         /**
+           * The only way to detect whether this object has a non-default
+           * implementation of |toString| is to check whether it returns
+           * '[object Object]'. Unfortunately, we cannot simply compare |arg.toString|
+           * and |Object.prototype.toString| as |arg| typically comes from another
+           * compartment.
+           */
+         if (argToString === "[object Object]") {
+           return JSON.stringify(arg);
+         } else {
+           return argToString;
+         }
        }
        return arg;
      };
 
      /**
       * A Shared LOG utility function that only logs when DEBUG is set.
       *
       * @params {string|object}

new file mode 100644
--- /dev/null
+++ b/toolkit/components/osfile/tests/xpcshell/test_logging.js
@@ -0,0 +1,64 @@
+"use strict";
+
+Components.utils.import("resource://gre/modules/osfile.jsm");
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+/**
+ * Tests logging by passing OS.Shared.LOG both an object with its own
+ * toString method, and one with the default.
+ */
+function run_test() {
+  do_test_pending();
+  let messageCount = 0;
+
+  // Create a console listener.
+  let consoleListener = {
+    observe: function (aMessage) {
+      ++messageCount;
+      //Ignore unexpected messages.
+      if (!(aMessage instanceof Components.interfaces.nsIConsoleMessage)) {
+        return;
+      }
+      if (aMessage.message.indexOf("TEST OS") < 0) {
+        return;
+      }
+
+      if(messageCount == 1) {
+       do_check_eq(aMessage.message, "TEST OS {\"name\":\"test\"}\n");
+      }
+      if(messageCount == 2) {
+        do_check_eq(aMessage.message, "TEST OS name is test\n");
+        // This is required, as printing to the |Services.console|
+        // while in the observe function causes an exception.
+        do_execute_soon(function() {
+          toggleConsoleListener(false);
+          do_test_finished();
+        });
+      }
+    }
+  };
+
+  // Set/Unset the console listener.
+  function toggleConsoleListener (pref) {
+    OS.Shared.DEBUG = pref;
+    OS.Shared.TEST = pref;
+    Services.console[pref ? "registerListener" : "unregisterListener"](
+      consoleListener);
+  }
+
+  toggleConsoleListener(true);
+
+  let objectDefault = {name: "test"};
+  let CustomToString = function() {
+    this.name = "test";
+  }
+  CustomToString.prototype.toString = function() {
+    return "name is " + this.name;
+  }
+  let objectCustom = new CustomToString();
+  OS.Shared.LOG(objectDefault);
+  OS.Shared.LOG(objectCustom);
+  // Once both messages are observed OS.Shared.DEBUG, and OS.Shared.TEST
+  // are reset to false.
+}
+

--- a/toolkit/components/osfile/tests/xpcshell/xpcshell.ini
+++ b/toolkit/components/osfile/tests/xpcshell/xpcshell.ini
@@ -1,7 +1,8 @@
 [DEFAULT]
 head =
 tail =
 
 [test_path.js]
 [test_osfile_async.js]
 [test_profiledir.js]
+[test_logging.js]