They do use SubtleCrypto digest [0] in secure contexts, which does the hashing natively.

Specifically for Firefox [1] they switch to the JavaScript fallback because that's actually faster [2] (because of overhead probably):

> One of the biggest sources of lag in Firefox has been eliminated: the use of WebCrypto. Now whenever Anubis detects the client is using Firefox (or Pale Moon), it will swap over to a pure-JS implementation of SHA-256 for speed.

[0] https://developer.mozilla.org/en-US/docs/Web/API/SubtleCrypt...

[1] https://github.com/TecharoHQ/anubis/blob/main/web/js/algorit...

[2] https://github.com/TecharoHQ/anubis/releases/tag/v1.22.0