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.