Firn densification is commonly modelled using 1D column approaches that neglect the deformation history associated with ice flow. Here, we present a flowline firn model that explicitly accounts for the accumulation and horizontal strain experienced by firn along its trajectory and use it to infer density-dependent rheology directly from observations.</p> <p>Applying the method to two ice-core sites in northeast Greenland with contrasting strain regimes, we constrain both the total densification response and its partitioning into shear and volumetric components. While steady-state and flowline inversions agree at large scales, the combined function governing vertical strain is tightly constrained, whereas individual rheological parameters remain poorly identifiable.</p> <p>The inferred rheology departs markedly from standard formulations, with enhanced near-surface densification, reduced viscous Poisson ratios, and pronounced non-monotonic structure. These behaviours are incompatible with a single power-law rheology and instead require the combined action of multiple deformation mechanisms, including grain-size-dependent creep and evolving anisotropy. The resulting parameters should therefore be interpreted as effective representations of these processes.</p> <p>These results further imply that firn-derived gas proxies such as &Delta;age and <em>&delta;</em>¹⁵N may contain a dynamical component linked to horizontal strain, with potential implications for the interpretation of ice-core chronologies.