'firmly-separated' originates from the combination of 'firmly' and 'separated', where 'firmly' means 'in a strong, secure manner' and 'separated' means 'divided or kept apart'.

'Firmly' and 'separated' have been used in English since the Middle Ages, with 'firmly' deriving from the Old French 'fermement' and 'separated' from the Latin 'separatus'.

Initially, 'firmly-separated' meant 'strongly divided', and this meaning has largely remained the same in modern usage.