To unlock fire,..... uhm just go to like unlockables and go to Fire Flasks, it will tell you what you need
i personally forgor how to get fire

This method will only work on CrazyGames or something else that supports advertisements.
Watch an ad to get 2x fire and 2x fuel burning speed for 30 minutes, and keep doing that!
When you have enough fire, Get a 2.5x fire upgrade, whenever possible! You can save up for things too. Just repeat this and move to steam if you want, and start doing method 2 whenever you want!

Gain fire as normal, and when you have enough fire to get auto-prestige, just wait on that! [before you have enough fire to get auto-prestige, you should do Method One because its free and just takes a little extra time, this is an idle game after all!]

"Buying every fire upgrade possible, and after getting all the fire auto upgrades (Auto Prestige, Auto Research etc) setup your crafting presets and turn them all on active to start auto prestiging/researching for fuel to make the burning timer shorter. When it takes too many prestiges between burn upgrades stop auto prestiging and wait until after buying 1 or 2 of the fuel gain upgrades (and other fire upgrades) then repeat. - Fuel Farming

This method will get super easy after unlocking all the ice auto upgrades later (5 sec prestige cooldown, 10x research, and all the auto unlocks/buys) at this point everything becomes automatic except when turning on and off auto prestige / freezing." - Reikoku

I am open to method suggestions, so post what method YOU used to get tons of fire in the comments! Who knows, it might even get added! [With credit, of course!]