The assessment and treatment of adults who set fires deliberately are underdeveloped relative to other areas of forensic-clinical psychology. From a scientist-practitioner perspective, all clinical assessment and treatment should be guided by a theoretical and empirically based understanding of the presenting clinical phenomena. In this paper, we critically review current typologies, motives, and theories regarding the etiological features of deliberate adult firesetting. Then, using a theory knitting perspective, we synthesize the prime parts of this information into a comprehensive multifactorial framework of deliberate firesetting. The resulting Multi-Trajectory Theory of Adult Firesetting (M-TTAF) is an integration of current theory, typological, and research knowledge into a comprehensive etiological theory of firesetting along with its maintenance, and desistence. In addition to this overall theoretical framework, we summarize five associated prototypical firesetting trajectories (or patterns of characteristics leading to the firesetting behavior) that stem from our theoretical work. We examine this new theory according to key evaluative components associated with theory construction and conclude by highlighting the M-TTAF's potential application in future research and practice innovation with adult firesetters.