CLEO 6.0 needs FEBE 8.0 or higher to work. If you have the proper versions installed and you still get a message about the wrong FEBE version, please install . I have been unable to duplicate this problem and am asking CLEO users to help debug.
When the message window pops up, you will see some debugging information. Please copy/paste that information in this thread.
With this version, you can also turn off the FEBE check by setting extensions.cleo.checkforFEBE to false in about:config.