Hi, how can I use lava in the "fires" configuration part to use a lava-block to heat the cualdron?`
I've already tried minecraft:lava and liquid:lava but no of them worked

Vanilla lava actually has two block names based on if it's "flowing" or not, see the Minecraft wiki article. You probably just need to add both.

Closing for lack of response, you are probably missing one of the vanilla block names