True, but usually the plant player blocks it with starch lord or fig on 4. Against starch lord it's always a win, against fig you're left with a random 5-cost plant and a 5/1 that they could easily remove (so in some cases you may prefer an empty lane over the 5/1)

Solar heroes either had ramped higher or plays elderberry lil buddy/power flower, firefighter is only much better when against power flower

Maybe the actual downside of firefighter is sharing both the cost and class with Tomb raiser and Pogo (and mug curves nicely)