Just discovered another #Firefox setting I need to toggle:
→ `intl.regional_prefs.use_os_locales` (from false to true)
so that the system-wise regional settings set to France (Français) are taken into account, meaning date pickers with DD/MM/YYYY instead of MM/DD/YYYY.
And no, that's not even about a discrepancy with the language, since that's set to English (United Kingdom) anyway…