With each new update of Firefox, the language setting of the menu-/taskbar changes back to the original English language. Not a problem when the standard language must be English, but this can be really annoying for non-English spoken Firefox users.
To get the correct language for Firefox, visit this page: https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/.
Select the correct version and double click on the folder.
For Linux click on the following folder: