The current link in this post goes to a year-old story about the online translation feature… here is the same site’s coverage of this week’s news - which is that there is now offline translation support: https://www.ghacks.net/2022/05/30/firefox-translations-firefoxs-offline-translate-feature-is-making-progress/ (i assume this is what OP actually meant to post). (edit: OP fixed the post’s link)

Here is a web page that loads their wasm translation engline and does the actual translation offline (and it does work in the stable release of Firefox). It’s irritating that the extension still requires a nightly firefox build, as I’d like to use it in my daily browsing but I don’t want to use nightly all the time.