Firn, the permeable and porous material through which snow compacts into glacier ice, plays an important role in glacier hydrology, mass balance, and dynamics. We investigated seasonal to decadal evolution of firn on the Juneau Icefield using field measurements and firn modeling, and found that the firn is thinning and warming. Our study offers insights into consequent changes in firn hydrology, including declining refreezing capacity and inter-seasonal shifts in liquid-water retention.