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What software is typically used to create a font like this? Are there any good open source options?

Glyphs.app provided a free license for Fira Code since font is open-source. I’m using it because original Fira Mono is developed in it as well.

Monoid uses FontForge (free)

The readme says "Fira Code is now drawn and built in Glyps 2 app (should improve compatibility)."