https://support.mozilla.org/en-US/kb/website-translation#w_how-do-i-stop-the-translation-panel-from-opening-automatically <blockquote>How do I stop the translation panel from opening automatically? When Firefox detects that a page is translatable, it will open the translation panel automatically. To turn this off, click the cog icon and select the '''Always offer to translate''' option to turn it off. Alternatively, you can choose to turn the panel off only for certain languages or sites by selecting '''Never translate [language]''' or '''Never translate this site''' respectively.</blockquote>

There is '''NO''' never translate (language) and "never translate this site" doesn't work since I must confirm this each time I'm visit the same site !