Firefox Quantum version (>=57) is available here: https://addons.mozilla.org/de/firefox/addon/google-translator-webextension/.
Auto-update did not work in my case, so search for: Google™ Translator (webextension).
These are 5 stars for the now obsolete addon-sdk API version that does not work anymore with FF57.
It is said that the new webextension version, which was published 7 (seven) months ago is even more stable.
Thank you Andy Portmen