Using after upgrade crashes Firefox

[jgrande]

Not completely sure about this, but it looks like if you install a new version of the addon with changes to the C bindings Firefox will crash the next time you invoke the extension.

The solution is to hear the onLoad event of the add-on and unload the old library.

See: https://developer.mozilla.org/en-US/Add-ons/SDK/Tutorials/Listening_for_load_and_unload