diff --git a/dom/system/OSFileConstants.cpp b/dom/system/OSFileConstants.cpp
index 6033741d4e5..ded54b7f503 100644
--- a/dom/system/OSFileConstants.cpp
+++ b/dom/system/OSFileConstants.cpp
@@ -380,6 +380,13 @@ static dom::ConstantSpec gLibcProperties[] =
   { "OSFILE_OFFSETOF_DIRENT_D_TYPE", INT_TO_JSVAL(offsetof (struct dirent, d_type)) },
 #endif // defined(DT_UNKNOWN)
 
+  // Under MacOS X, |dirfd| is a macro rather than a function, so we
+  // need a little help to get it to work
+#if defined(dirfd)
+  { "OSFILE_SIZEOF_DIR", INT_TO_JSVAL(sizeof (DIR)) },
+
+  { "OSFILE_OFFSETOF_DIR_DD_FD", INT_TO_JSVAL(offsetof (DIR, __dd_fd)) },
+#endif
 
   // Defining |stat|
 
diff --git a/toolkit/components/osfile/osfile_unix_back.jsm b/toolkit/components/osfile/osfile_unix_back.jsm
index 7a56ac7ab0d..720b7186b12 100644
--- a/toolkit/components/osfile/osfile_unix_back.jsm
+++ b/toolkit/components/osfile/osfile_unix_back.jsm
@@ -108,19 +108,6 @@
        Types.time_t =
          Types.intn_t(OS.Constants.libc.OSFILE_SIZEOF_TIME_T).withName("time_t");
 
-       Types.DIR =
-         new Type("DIR",
-                  ctypes.StructType("DIR"));
-
-       Types.null_or_DIR_ptr =
-         Types.DIR.out_ptr.withName("null_or_DIR*");
-       Types.null_or_DIR_ptr.importFromC = function importFromC(dir) {
-         if (dir == null || dir.isNull()) {
-           return null;
-         }
-         return ctypes.CDataFinalizer(dir, _close_dir);
-       };
-
        // Structure |dirent|
        // Building this type is rather complicated, as its layout varies between
        // variants of Unix. For this reason, we rely on a number of constants
@@ -175,6 +162,37 @@
          Types.stat = stat.getType();
        }
 
+       // Structure |DIR|
+       if ("OSFILE_SIZEOF_DIR" in OS.Constants.libc) {
+         // On platforms for which we need to access the fields of DIR
+         // directly (e.g. because certain functions are implemented
+         // as macros), we need to define DIR as a hollow structure.
+         let DIR = new OS.Shared.HollowStructure(
+           "DIR",
+           OS.Constants.libc.OSFILE_SIZEOF_DIR);
+
+         DIR.add_field_at(
+           OS.Constants.libc.OSFILE_OFFSETOF_DIR_DD_FD,
+           "dd_fd",
+           Types.fd.implementation);
+
+         Types.DIR = DIR.getType();
+       } else {
+         // On other platforms, we keep DIR as a blackbox
+         Types.DIR =
+           new Type("DIR",
+             ctypes.StructType("DIR"));
+       }
+
+       Types.null_or_DIR_ptr =
+         Types.DIR.out_ptr.withName("null_or_DIR*");
+       Types.null_or_DIR_ptr.importFromC = function importFromC(dir) {
+         if (dir == null || dir.isNull()) {
+           return null;
+         }
+         return ctypes.CDataFinalizer(dir, _close_dir);
+       };
+
        // Declare libc functions as functions of |OS.Unix.File|
 
        // Finalizer-related functions
@@ -241,10 +259,19 @@
                     /*return*/ Types.negativeone_or_fd,
                     /*fd*/     Types.fd);
 
-       UnixFile.dirfd =
-         declareFFI("dirfd", ctypes.default_abi,
-                    /*return*/ Types.negativeone_or_fd,
-                    /*dir*/    Types.null_or_DIR_ptr);
+       if ("OSFILE_SIZEOF_DIR" in OS.Constants.libc) {
+         // On platforms for which |dirfd| is a macro
+         UnixFile.dirfd =
+           function dirfd(DIRp) {
+             return Types.DIR.in_ptr.implementation(DIRp).contents.dd_fd;
+           };
+       } else {
+         // On platforms for which |dirfd| is a function
+         UnixFile.dirfd =
+           declareFFI("dirfd", ctypes.default_abi,
+                      /*return*/ Types.negativeone_or_fd,
+                      /*dir*/    Types.DIR.in_ptr);
+       }
 
        UnixFile.chdir =
          declareFFI("chdir", ctypes.default_abi,