The Firma 150a has a small gray wire that is used for the Smart features. You can see it in the middle of each of the connectors between the positive and negative leads. I just snipped mine off at the connector and heat shrinked them to the cables before I put braided sleaving on.
Snip them off at the plug and heat shrink them to the cables. They are for the Spektrum Smart features which you will not be able to use with the Traxxas batteries anyway.