'firmly-built' originates from the combination of 'firmly' and 'built', where 'firmly' is derived from the Old English 'fyrm' meaning 'strong' and 'built' from the Old English 'byldan' meaning 'to construct'.

'firmly' changed from the Old English 'fyrm' to the modern English 'firmly', and 'built' evolved from 'byldan' to 'built'.

Initially, 'firmly' meant 'strongly' and 'built' meant 'constructed', and this meaning has largely remained the same in modern usage.