nsISeekableStream in Firefox 3.0

  seek

  This method moves the stream offset of the steam implementing this
  interface.

   @param whence specifies how to interpret the 'offset' parameter in
                 setting the stream offset associated with the implementing
                 stream.
     
   @param offset specifies a value, in bytes, that is used in conjunction
                 with the 'whence' parameter to set the stream offset of the 
                 implementing stream.  A negative value causes seeking in 
                 the reverse direction.

  setEOF

  This method truncates the stream at the current offset.

  tell

  This method reports the current offset, in bytes, from the start of the 
  stream.