A fork of marian-dev[1] is the underlying machine-translation engine:

- https://github.com/browsermt/marian-dev

Development of higher-level code wrapping around marian-dev make suitable for the browser-extension happens at:

- https://github.com/browsermt/bergamot-translator

Some of the WebAssembly optimizations are available in bergamot-translator/marian-dev. Rest are in Firefox source-code. A start point could be https://bugzilla.mozilla.org/show_bug.cgi?id=1720747.

Extension code is open-source, and linked already in other comments: - https://github.com/mozilla/firefox-translations

[1] https://github.com/marian-nmt/marian-dev