unpacking firefox extension on deployment

Before Firefox 4 all extensions were extracted when installed. This behavior changed, the new default is to keep the extension as packed .xpi file for performance reasons. Extensions can still opt into the old behavior by specifying unpack flag in install.rdf

<Description about="urn:mozilla:install-manifest">
   <em:id>extension@mysite.com</em:id>
   <em:unpack>true</em:unpack>
   ...
</Description>