The method developed previously by the present authors on the basis of the quasiparticle random-phase approximation with allowance for a single-particle continuum is generalized to the case of finite temperatures and is implemented numerically for the isovector E1 resonance in the nuclei of even tin isotopes-specifically, in the stable nucleus ¹²⁰Sn and in the unstable nucleus ¹⁰⁴Sn. The temperature dependence of the integrated features of the resonance and the temperature dependence of its envelope are discussed.