Securely (6)
'securely' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for firmly that I've seen before include "fast" , "Resolutely" , "Strongly" , "With resolute determination" .)
(Other definitions for safely that I've seen before include "In a manner not exposed to danger" , "In a secure manner" , "Without danger" , "In a risk-free manner" .)