Please use this filter definition ( with required changes ) for firstname column.
filter: { crules: [{condition: 'contain' }], listener: function(evt, ui){
debugger;
var filterRules = [{ dataIndx: 'firstname', condition: 'contain', value: ui.value},
{ dataIndx: 'lastname', condition: 'contain', value: ui.value}];
this.filter({
oper: 'replace',
rules: filterRules
});
}}