The Collaborative International Dictionary of English v.0.48:

Firmly \Firm"ly\, adv.
   In a firm manner.
   [1913 Webster]

WordNet (r) 3.0 (2006):

firmly
    adv 1: with resolute determination; "we firmly believed it";
           "you must stand firm" [syn: firm, firmly,
           steadfastly, unwaveringly]
    2: in a secure manner; in a manner free from danger; "she held
       the child securely" [syn: securely, firmly]
    3: with firmness; "held hard to the railing" [syn: hard,
       firmly]

Moby Thesaurus II by Grady Ward, 1.0:

108 Moby Thesaurus words for "firmly":
   adamantly, bec et ongles, bona fide, close, closely, committedly,
   compactly, consistently, constantly, deafeningly, decidedly,
   decisively, densely, determinedly, devotedly, dourly, earnestly,
   faithfully, fast, firm, fixedly, forcefully, forcibly, grimly,
   hammer and tongs, hardly, heart and soul, heartily, heavily,
   immovably, immutably, imperviously, implacably, impliably,
   impregnably, in all seriousness, in earnest, in good earnest,
   in good faith, inexorably, inflexibly, inseparably, intensely,
   intransigently, invincibly, invulnerably, ironhandedly,
   irreconcilably, irresistibly, like a bulldog, like a leech, loudly,
   loyally, lustily, mightily, obdurately, obstinately, perseveringly,
   persistently, powerfully, pungently, purposefully, relentlessly,
   resistantly, resolutely, resolvedly, responsibly, rigidly,
   rigorously, robustly, ruggedly, securely, seriously, sincerely,
   singlemindedly, solid, solidly, soundly, stalwartly, staunchly,
   steadfastly, steadily, sternly, stiffly, stoutly, strongly,
   sturdily, tenaciously, thick, thickly, tight, tightly,
   tooth and nail, unalterably, unbendingly, unchangeably,
   uncompromisingly, unregenerately, unrelentingly, unswervingly,
   unwaveringly, unyieldingly, vigorously, wholeheartedly,
   with a will, with constancy, with good faith,
   with total dedication