Fixed (hopefully) this warning from the mozilla add-on validator:
  `nsITransferable` has been changed in Gecko 16.

  Warning: The `nsITransferable` interface has changed to better support i
  Private Browsing Mode. After instantiating the object, you should call
  the `init` function on it before any other functions are called.
  See https://developer.mozilla.org/en-US/docs/Using_the_Clipboard for more information.

This was in the code for 'copy'.
Added comments explaining some of the warnings from the mozilla add-on validator.
Removed some unused code for saving images in PNG format.
Simplified fix for Fix #108: TZP stops working on Nightly
Version 2.4rc2.  Compatible with ff 21.0a1