Does not work. Switching reloads the page but the Accept-Language header sent does not change at all. Firefox continues to use what is configured. Perhaps this is just out of date in 2026.

Works for me, needed it to stop websites from translating themselves and it did just that, so thanks.

Does not work

Impossible to add a new language. There is no save button in the settings.

Doesn't do anything. I.e. navigator.language does not change.

Chinese Traditional 竟然是五星旗

seems it does not work front end javascript. tried with tempus dominus library but no luck so far.

Does the job. Go to Firefox preferences then Add-ons to add/remove languages.

Excellent. Does what it says and does it simply, but well. Highly recommended.