A new approach to characterize airborne firebrands during Wildland-Urban Interface (WUI) fires is detailed. The approach merges the following two imaging techniques in a single field-deployable diagnostic tool: (1) 3D Particle Tracking Velocimetry (3D-PTV), for time-resolved mapping of firebrand 3D trajectories, and (2) 3D Particle Shape Reconstruction (3D-PSR), to reconstruct 3D models of individual particles following the Visual Hull principle. This tool offers for the first time the possibility to simultaneously study time-resolved firebrand fluxes and firebrand size distribution to the full extent of their three-dimensional nature within a control volume. Methodologies used in the present work are presented and their technical implementation is discussed. Validation tests to confirm proper tracking/sizing of particles are detailed. The diagnostic tool is applied to a firebrand shower artificially generated at the NIST National Fire Research Laboratory. A novel graphic representation, that incorporates both the Cumulative Particle Count (CPC, particles m−2) and Particle Number Flux (PNF, particles m−2 s−1) as relevant exposure metrics, is presented and the exposure level is compared to that of an actual outdoor fire. Size distributions obtained for airborne firebrands are compared to those achieved through ground collection and strategies to improve the particle shape reconstruction method are discussed.