We demonstrated a femtosecond mode-locked Er:ZrF₄-BaF₂-LaF₃-AlF₃-NaF (Er:ZBLAN) fiber laser at 2.8 μm based on the nonlinear polarization rotation technique. The laser generated an average output power of 317 mW with a repetition rate of 107 MHz and pulse duration as short as 131 fs. To the best of our knowledge, this is the shortest pulse generated directly from a mid-infrared mode-locked Er:ZBLAN fiber laser to date. Numerical simulation and experimental results confirm that reducing the gain fiber length is an effective way to shorten the mode-locked pulse duration in the Er:ZBLAN fiber laser. The work takes an important step towards sub-100-fs mid-infrared pulse generation from mode-locked Er:ZBLAN fiber lasers.