diff --git a/netwerk/base/public/nsIFileStreams.idl b/netwerk/base/public/nsIFileStreams.idl
index 11f5aa0d4fe8..5c0893e3e8a5 100644
--- a/netwerk/base/public/nsIFileStreams.idl
+++ b/netwerk/base/public/nsIFileStreams.idl
@@ -48,8 +48,11 @@ interface nsIFileInputStream : nsIInputStream
 {
     /**
      * @param file          file to read from (must QI to nsILocalFile)
-     * @param ioFlags       file open flags listed in prio.h
-     * @param perm          file mode bits listed in prio.h
+     * @param ioFlags       file open flags listed in prio.h (see
+     *                      PR_Open documentation) or -1 to open the
+     *                      file in default mode (PR_RDONLY).
+     * @param perm          file mode bits listed in prio.h or -1 to
+     *                      use the default value (0)
      * @param behaviorFlags flags specifying various behaviors of the class
      *        (see enumerations in the class)
      */
@@ -87,9 +90,13 @@ interface nsIFileInputStream : nsIInputStream
 interface nsIFileOutputStream : nsIOutputStream
 {
     /**
-     * @param file          - file to write to (must QI to nsILocalFile)
-     * @param ioFlags       - file open flags listed in prio.h
-     * @param perm          - file mode bits listed in prio.h
+     * @param file          file to write to (must QI to nsILocalFile)
+     * @param ioFlags       file open flags listed in prio.h (see
+     *                      PR_Open documentation) or -1 to open the
+     *                      file in default mode (PR_WRONLY |
+     *                      PR_CREATE_FILE | PR_TRUNCATE)
+     * @param perm          file mode bits listed in prio.h or -1 to
+     *                      use the default permissions (0664)
      * @param behaviorFlags flags specifying various behaviors of the class
      *        (currently none supported)
      */