Keep it simple
As an author I've learned that short and sweet explanations are the best kind, the more you tend to complicate things the less attention your audience is willing to provide you with. I would suggest not delving into the finer details of chemistry unless your story explicitly revolves around a character that's actively trying to break the secret of Dragon fireballs to utilize them as the next nuclear option.
Firebreathing Dragons simply breathe fire, that's it, nobody wants to know from what kind of matter is that breath composed and they don't care. Firebreathing dragons are commonplace, they're simple, they make sense fantasy-wise and they're cool because of it. In some fiction they flat-out breathe fire the moment they open their mouths and make an aggressive hiss, in others they breathe flammable vapor they can manipulate in certain ways and then provide a spark that lights that vapor (most obvious example in How To Train Your Dragon series). They just do it, it's their nature and pretty much nobody cares about the "how" part.
Explosive fireball spitting dragons sounds new to me, I'm actually considering using the idea if I ever get around to implementing dragons into my world. They could be simply spitting a slime that becomes particularly unstable once exposed to open air and it violently explodes the moment it hits any sort of barrier. For this to work and obviously for it not to be dangerous for the dragon spitting it, it goes without saying that this slime should work as an equivalent of a modern RPG missile, meaning it requires to travel a certain distance or spent at least 1-2 seconds in open air before it actually arms itself by spontaneously igniting and then violently exploding when it hits anything afterwards.
The slime itself could be produced naturally by the dragon, probably by an additional set of glands that work in similar ways to the spit-glands. Heck, the dragon's spit itself could be that very slime, meaning it can be harvested but has to be done when the dragon is alive since a dead dragon clearly cannot produce slime. This makes it not only difficult to obtain but ludicrously dangerous once it is obtained so handling the slime by anyone other than the dragon is a pretty risky thing to do, basically a high risk high reward situation.
