Originally a line from Wilt thou, thus unkind, reave me? It was popularized by Zheanna Erose, a YouTuber who created the channel Trans Voice Lessons. She offered it as a mantra for feminizing voice practice in several videos, and as she is the premier transfeminine voice trainer, it spread everywhere. (If you've ever tried to learn how to sing or how to change your voice, you might be familiar with how much magical bullshit is in common terminology, and how incredibly unhelpful it is. Z uses a more scientific approach to her training that is also very effective, making her the automatic gold standard.)
In this video, Zheanna explains that she found the phrase by accident after being hired to read that poem with a feminine voice gliding smoothly into a masculine one, which she found difficult. From there, she started using "heat from fire, fire from heat" before gaming to feminize her voice.
Starting at 2:08 in the linked video, she explains in detail why the mantra works for voice training. For example, the 't' in heat is supposed to be a glottal stop, which contrasts with the breathy 'h' onset in a way that can highlight unwanted constriction or other bad stuff. Much of her explanation of the mantra's utility is that it combines a lot of tricky exercises into one phrase, which would seem counter to the idea that it's naturally feminizing. But it is, though.
While Z is herself a big memer, I'm not sure how much she directly influenced the spread of the phrase beyond using it in voice work. (She's personally worked with hundreds, if not thousands of people, and some of them spread her ideas to others, in addition to the supremacy of her YouTube channel.) A representative meme has one transfemme seeing someone they suspect is also trans and saying the phrase "heat from fire" to check. The response was "fire from heat", and presumably they dated after that, but I don't think it was included in the meme.
It's been included in stories such as Chapter 16 of Zoe Storm's To Own the Libs and it was the title of a game by Anna Anthropy about a witch summoning Asphodel for sexy times. highly recommended, very nsfw
There is definitely more out there.