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The effect of thermal shape fluctuations on the recurrent fluorescence of boron cluster cations, B$_N^+$ ($N=9-14$), has been investigated numerically, with a special emphasis on B$_{13}^+$. For this cluster, the electronic structures of the ground state and the four lowest electronically excited states were calculated using time-dependent density functional theory, and sampled on molecular dynamics trajectories of the cluster calculated at an experimentally relevant excitation energy. The sampled optical transition matrix elements for B$_{13}^+$ allowed to construct its emission spectrum from the thermally populated electronically excited states. The spectrum was found to be broad, reaching down to at least 0.85 eV. This contrasts strongly with the static picture, where the lowest electronic transition happens at 2.3 eV. The low-lying electronic excitations produce a strong increase in the rates of recurrent fluorescence, calculated to peak at $4.6 \times 10^{4}$ s$^{-1}$, with a time-average of $8 \times 10^{3}$ s$^{-1}$. The average value is one order of magnitude higher than the static result, approaching the measured radiation rate. Similar results were found for the other cluster sizes. Furthermore, the radiationless crossing between the ground-state and the first electronic excited state surfaces of B$_{13}^+$ was calculated, found to be very fast compared to experimental time scales, justifying the thermal population assumption.