IR-visible upconversion fluorescence spectroscopy and thermal effects in Pr³⁺/Yb³⁺-codoped Ga₂O₃:La₂S₃ chalcogenide glasses excited at 1.064 µm is reported. Intense visible upconversion emission in the wavelength region of 480-680 nm peaked around 500, 550, 620 and 660 nm is observed. Upconversion excitation of the Pr³⁺ excited-state visible emitting levels is achieved by a combination of phonon-assisted absorption, energy-transfer and phonon-assisted excited-state absorption processes. A threefold upconversion emission enhancement induced by thermal effects when the codoped sample was heated in the temperature range of 20-200 °C is demonstrated. The thermal-induced enhancement is attributed to a multiphonon-assisted anti-Stokes process which takes place in the excitation of the ytterbium and excited-state absorption of the praseodymium. The thermal effect is modelled by conventional rate equations considering temperature-dependent effective absorption cross-sections for the ²F_7/2-²F_5/2 ytterbium transition and ¹G₄-³P₀ praseodymium excited-state absorption, and it is shown to agree very well with experimental results. Frequency upconversion in singly Pr³⁺-doped samples pumped at 836 nm and 1.064 µm in a two-beam configuration is also examined.