Firefox already supports Bing and Yandex Translation engines, forget about Yandex, even Microsoft Translator feature hasn’t made to the stable version, now, Mozilla has added support for using Google Translation API which you shouldn’t need to get excited about.
The feature may never ship like Bing and Yandex, but Firefox team hopes even though if they can’t ship the feature with an API key to release channel, it will help someone who wants to try to sign up and receive their own key. ICYDK, Google Translation API isn’t free, you need to pay to use it.
We’ve noticed browser.translation.engine preference has been set to Google in Firefox 64 Nightly. We toggled the below preferences and ensured ‘Translate Web Content’ is checked under Language in General section of options, to see Google Translate feature works, it has not, the info bar has appeared for a German site, but the translation failed, we received an error on our end.
If you’re missing native Translation feature Google offers in Chrome, you can use S3.Translator add-on in Firefox.
Which add-on are you using in Firefox for Translation of pages?