That is the new web translation feature.

• https://support.mozilla.org/en-US/kb/website-translation

You can set this pref to false in about:config to prevent opening the translation pop-up panel.

• about:config => browser.translations.automaticallyPopup => false

You can control this translation feature via:

• about:config => browser.translations.enable => true or false

Note that you do not get the translation icon for pages that are in the preferred language for displaying pages list, in this case use the "3-bar" Firefox menu button.

My request was not to know how Firefox translation works, I’m asking how to make the translation add on off !

• Go to about:config and set browser.translations.automaticallyPopup => false

Did you read the second part in my above reply about the two prefs that control this translation feature?

Yes I did. Unfortunately, maybe I didn’t well understood , but it didn’t work ! Firefox translate logo still there and still offering translation !

Try out browser.translations.enable then.