You will earn firestones through Prestige. Firestones gives you gold bonus, and allows you to buy Fire Tokens for research. First time you should prestige at around 100 firestones. After that you get automatically a popup to prestige whenever you have a 2x multiplier. This means that the amount of firestones that you have doubles when you prestige. The newly ‘collected’ firestones will be added on top of the old firestones. But you don’t have to prestige at 2x multiplier. You can wait longer and get a higher multiplier. Do remember that you have to collect 6 times as much gold to double the amount of newly collected firestones. So it’s better to do shorter runs than longer runs. But stay at least an hour at your farm stage.
You will also run into prestige walls as I call them. When you don’t get any gold bonus through other features in the game, eventually the time it takes to do a 2x prestige will take longer and longer. Can be 8 hours or longer before you start seeing a 2x multiplier. This will greatly slow down stage and character level progression of course. To fix this you have to find ways to increase gold bonus. This can be achieved by making your heroes stronger, allowing you to farm higher stages (x1.25 gold per stage). Obtaining new rings will help greatly. Enchanting rings. Level up or evolve the dragon. Research or upgrade Gold Bonus or Prestigious.
It’s best to use Prestige Tokens when you have reached one of these walls. That is when you get the most out of the prestige tokens. It may sound weird, you might think they are better used when you have a very high multiplier, but that’s wrong.