1) Closely 2) Definitely 3) Downright 4) Emphatically 5) Firm 6) Forcefully 7) In a manner free from danger 8) In a secure manner 9) In a secure way 10) Positively 11) Securely 12) Soundly 13) Stably 14) Steadfastly 15) Steadily 16) Strongly 17) Sturdily 18) Unconditionally 19) Unwaveringly 20) With assurance 1) Relentlessly 2) Securely 3) Severely 4) Steadfastly 5) Strictly 6) Strongly 7) Surely 8) Thoroughly 9) Tight 10) Unwaveringly 
• (adv.) In a firm manner.
Firm'ly adverb In a firm manner. 
firmemente, con firmeza