Improve bookmarks backend

I'm currently trying to write a tool to synchronize bookmarks between different browsers and improvements in the following area would be nice:

  • change the format of bookmarks.html to a more modern format (xbel, xml)
  • move favicons store to another cache file, the bookmarks file is hardly the right place to store them
  • on the fly changes of bookmarks.html should be possible. This would allow external programs and services (SyncServices on OS X) to change the bookmarks file without restarting Firefox