The Thunderbird is a convertible-only (optional hard-top). And no, it is not available at this time.
I really wish Hertz offered the Volvo C70 Coupe and C70 Convertible as part of the Prestige Collection (they offer the S80/S60 Sedan and XC70 wagon). I have nothing against Mustang, but I'd much prefer the Volvo for my coupe and convertible rentals (I usually rent Lincoln TC/LS/Nav at the moment).