As bhackett confirmed, you're right about recording at the system library call level. I wasn't sure if it was more of an analogy or only referred to a version of Replay targeting backend servers written in other languages like Go, especially since the author mentioned hooking into the JS runtime in https://medium.com/replay-io/effective-determinism-54cc91f56.... But it looks like I misunderstood, and their browser product is their generic record/replay library integrated into Firefox, rather than a reimplementation of the same concepts.