When you first installed Firefox, did you download the en-US version or did you just select the language version that was recommended to you?

You may want to try redownloading and installing Firefox from the Mozilla website, but make sure that you select the desired language.

If you have Firefox in your local language and use a language pack to get another language like English then it is possible that the language pack doesn't update automatically. In that case you need to trigger this update via the about:addons page (Languages) or if you have no need for the local version, install Firefox in your preferred language like posted above.

• Bug 1566084 - (lang-updates) Language packs are not updated when Firefox is updated

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)