 Frontier Group Holdings Inc. ULCC, 3.76 said Monday its offer for Spirit Airlines Inc. SAVE, 4.23 is its last, best and final offer, and said it will not be making any further modifications to the financial terms. In a letter to Spirit management, Frontier CEO Barry Biffle said he continues to believe that the counteroffer from JetBlue Airways Corp. JBLU, -2.26 is less attractive than the Frontier proposal. 