"Helpi" in the sense of making someone’s task or work easier can be used with accusative or with the preposition "al".
In the sense of having a remedy for something (Esperanto: kontraŭ io) for somebody the accusative is preferred.
In the sense of being useful or effective you can exchange the preposition "al" and "kontraŭ" to get the positive and negative meaning. This is the case here. You could easily change it to: "La vento helpis kontraŭ la fajro." Therefore the "al" is preferred here. Surely, "… helpis la fajron" is not totally wrong.
So, if I would say: La X-o Y-as kontraŭ la Z-o. I could also say: La X-o Y-as al la Z-o? Or does this only work with "helpi"?
I've certainly seen Y-as al Z-o before (Li diris al mi. Mi donis al ŝi. ktp.) but what I read you saying here is that I can also turn those sentences around. (Li diris kontraŭ mi, Mi donis kontraŭ ŝi, ktp.)
Certainly, "... helpis la "
"Certainly" states something that is known for an objective fact, "surely" states an impression or opinion. Here it implies it's your impression or opinion that this is true, but you don't know for certain. Which is surely not the case. ;)
because in latin, russian, and german "help" is always used with the dative case. ( al has the same meaning as the dative case ) Latin has it's deep reasons to do this, but in german and russian it is just a leftover thing from the times of more complex grammar. In esperanto we should just say "helpis fajron" because there is no reason to say "helpis al fajro" other than mimicking german which we should not do!