Checkout source:
svn co https://svn.erp5.org/repos/public/erp5admin/trunk/ erp5admin
Build xpi:
cd erp5admin
make
Install erp5admin.xpi
Restart Firefox.

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.