The site manager is now able to browse local dirs, create new dirs, delete files and dirs. It took more time than expected because of the following problems:

  1. don't try to directly set the cursor on a XUL page element... The result can be sometimes weird: cursor is set but some XUL elements become unresponsive
  2. Windows sucks... And I found at least bug 239804 to prove it. It took me quite a while to figure out why the contents of the directory were correctly removed but never the directory itself.