Seriously, why isn’t it? Why switch the e and the r for that one word? And if "firey" is no good, then why don't we just replace the e with a y, like in wiry? Is there a reason?
Fire didn’t always used to be spelled fire. It comes from the Old English fyr (I think it would rhyme with Tyr) and when Old became Middle, they changed a lot of vowels around and the y became ie. For a few centuries, anyway.
Before about 1200 CE, the word was spelled fier (okay, it’s starting to make more sense now) Then, Old English gave way to Middle English. For about four hundred years, there was a war between the spellings, because really, in the sixteenth century people just spelled things the way they felt like it. Eventually, people started collaborating on universal spelling for English and "Fire" emerged as the preferred choice, which lasted into our Modern English language. But fier remained alive in the adjective form of the word. Which for some reason, they did not change.
Dang. I hate it when there’s a logical explanation. Well, mostly logical. Language evolution is weird.