abslibre/libre/iceweasel/iceweasel.install

share_extensions()
{
  mkdir -p usr/lib/firefox/browser/extensions

  if   [[ -d usr/lib/iceweasel/browser/extensions ]]
  then mv usr/lib/iceweasel/browser/extensions/* usr/lib/firefox/browser/extensions/
       rmdir usr/lib/iceweasel/browser/extensions
       ln -s ../../firefox/browser/extensions usr/lib/iceweasel/browser/extensions
  fi
}

pre_install()  { share_extensions ; }
post_install() { share_extensions ; }