From Snopes -
"In fairness, it has also been argued that Key may have intended the phrase as a reference to the British Navy’s practice of impressment (kidnapping sailors and forcing them to fight in defense of the crown), or as a semi-metaphorical slap at the British invading force as a whole (which included a large number of mercenaries), though the latter line of thinking suggests an even stronger alternative theory — namely, that the word “hirelings” refers literally to mercenaries, and “slaves” refers literally to slaves. It doesn’t appear that Francis Scott Key ever specified what he did mean by the phrase, nor does its context point to a single, definitive interpretation."