It's OK I've found the language button

No it's not OK. It automatically goes back to English. I need a french version, that offers me "Nouvel Onglet" not "New Tab" for example

If the Firefox user interface (toolbars) is in the wrong language or if you want to change the current language then download Firefox in the language of your choice.

You can find the full version of the current Firefox release (62.0) in all languages and all operating systems here:

• https://www.mozilla.org/en-US/firefox/all/
• https://www.mozilla.org/en-US/firefox/62.0/releasenotes/