This is a semi guess here but I imagine before the "outline" property was accepted as a standard that FireFox used -moz-outline for its CSS definition.
This happens a lot with latest tech that hasn't made it into the CSS spec yet like:
border-radius: 10px;
-moz-border-radius: 10px;
-webkit-border-radius: 10px;
Eventually border-radius will make it into the spec however until then both webkit and mozilla have provided their own css property
