Cool game (pun intended). Very polished and complete for a jam game, pretty impressive.
They main complaint I have is it is a bit hard to see the spikes (fire and ice on ground), they are very hard to spot before they activate (and maybe they could be more visible while active too). So sometimes I would plan a path and start, just to be caught off guard by a sneaky spike growing under my feet when i change state. The frustration is even bigger in ice spikes glued to ice blocks, where you don't have much contrast to differentiate a thing that kills from a thing that you need to be near.
One improvement that would make the mechanic easier to understand is to have a different visual for each temperature 'level'. When you light up on fire, you can melt 2 ice blocks, but you don't see any change in the state of your character midway. Let's say when you are in full fire, you have the current visual, but after melting one, you get a bit smaller and more red than yellow, and then, if you melt another one, you become a regular stone again. This would communicate better the 'resource' of temperature, and usually these kind of feedback are very welcome in puzzle games, makes the mechanic easier to understand and easier to read at a glance (now you gotta memorize: "oh, I already melted one block, so in the next, I'll be back to stone")
Another tiny worry i have is that I don't feel the theme of the jam in the game. Sure, thermometric 'scales', but you don't even see a scale in the game or anything like that
Other than these small gripes, the game is great! Gorgeous, feels nice to play and have some cool puzzles to solve, with well presented mechanics and twists.