Fira Mono.

I saw that some of the fonts had a ligature for === making it a long congruence sign instead of three equals signs, and I avoided those like the plague.

Same.

I was surprised that both Fira Code and Fira Mono were options, that was a bit cheeky.

Same here. I've never felt tempted to use another font since I adopted it--well, except for the Nerd Font variant, of course.