Hi, Now with Firefox ESR 52.6.0esr-2, I did some other tests, each time with a fresh profile:
1. It seems that language header is not always sent (as reported by websites that display headers) 2. Giving a default setting (with any language) in a .js file, while being applied in about:config, does not help. Example: pref("intl.accept_languages", "fr,es"); 3. But enforcing a setting the same way, while not nice for the user, works. Example: lockPref("intl.accept_languages", "fr,es"); Thanks for your work, Yvan
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