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.

As noted, this is an old article. You can install the plugin here: https://addons.mozilla.org/en-GB/firefox/addon/firefox-translations/

I just tried it on https://www.tagesschau.de/wirtschaft/technologie and it’s definitely good enough to be usable, although it has translated the top story as “What the new data glasses from Apple can”. Google Translate’s version is almost the same for most of it, although it gets “can do” right.

It initially recognised that it could translate feddit.de but seems to have stopped now. Hmm.

Anyway, even though German->English is a pretty easy test given that English is a Germanic language, I’m happy to leave it installed and test it in the wild.