Navigate to
http://svn.erp5.org/*checkout*/erp5/trunk/utils/erp5admin/erp5admin.xpi?content-type=application%2Fx-xpinstall
Install extension.
Restart Firefox.
Done.

If you want to customize menu, fetch sources from SVN:
  https://svn.erp5.org/repos/public/erp5/trunk/utils/erp5admin/

Then edit /content/erp5admin/erp5admin.xul file to your liking;
then run "make" and repeat the above procedure.