My XPI packs have the following structure:
----------------
install.js
install.properties
chrome/
        *.jar
defaults/
          profile/
                  <XX>/
                       <files for default profile>
          messenger/
                    <XX>/
                          mailViews.dat
myspell/
         aa-XX.aff
         aa-XX.dic
         LICENSE.TXT   (this is for the dictionaries)
searchplugins/
               <search-engine-icon>.gif
               <search-engine-definition>.src
----------------
XX is a region code, aa is a language code.

So you see that install.properties is added, containing the strings used 
by install.js - and the bin/ directory disappeared, chrome/, defaults/ 
and searchplugins/ are directly in the main folder.

in chrome/ you have the translated files for language and content 
pack, defaults/profile/<XX>/ contains the files to use for the 
default profile when it's created with your content pack, and
searchplugins may contain local search engines that should be available
to people using your content pack (e.g. German Google or similar).