Objective Firesetting in children is thought to be an indicator of severe conduct problems in young people. However, no research has examined whether childhood firesetting is also associated with increased risk of externalizing and suicidal behaviors in adulthood. Method Data were obtained from a longitudinal study (n = 1265). Childhood firesetting/conduct problems (7–10 years) were derived from an assessment of antisocial behavior. Externalizing/suicidal behavior was derived from the Composite International Diagnostic Interview and the Self-Report Delinquency Inventory. Generalized estimating equation (GEE) models estimated associations between childhood firesetting and adult substance use disorders, criminal offending, and suicidal ideation, adjusting for childhood conduct problems and other confounding factors. Associations between childhood and adult firesetting (age 18–40 years) were examined using cross-tabulation (χ2). Results Five percent of children reported firesetting (7–10 years). Childhood firesetting appeared to increase the risk of adult firesetting; however, in most cases adult firesetting was not associated with childhood firesetting (χ2 (1) = 4.15, p = .0417). Childhood firesetting was a risk marker for adult externalizing/suicidal behavior; however, the effect was relatively weak (IRR = 1.51; 95% CI: 1.11–2.05). Children with conduct problems who also engaged in firesetting were found to be at substantially higher risk of later externalizing/suicidal behavior (IRR = 2.84; 95% CI: 1.24–6.49). Conclusion This study found that childhood firesetting is a risk marker for adult externalizing/suicidal behavior, not an independent risk factor. It may be more useful for clinicians to focus on child conduct problems generally, rather than focussing on firesetting behavior.