The Collaborative International Dictionary of English v.0.48:

Headfirst \Head`first"\ (h[e^]d"f[~e]rst`), Headforemost
\Head`fore"most`\ (h[e^]d`f[=o]r"m[=o]st`), adv.
   With the head foremost; -- of motion.
   [1913 Webster]

WordNet (r) 3.0 (2006):

headfirst
    adv 1: with the head foremost; "the runner slid headlong into
           third base" [syn: headlong, headfirst]
    adj 1: with the head foremost; "a headfirst plunge down the
           stairs"; "a headlong dive into the pool" [syn:
           headfirst, headlong]

Moby Thesaurus II by Grady Ward, 1.0:

37 Moby Thesaurus words for "headfirst":
   a corps perdu, abruptly, carelessly, desperately, furiously,
   happen what may, hastily, head over heels, headforemost, headlong,
   heedlessly, heels over head, helter-skelter, holus-bolus,
   hotheadedly, hurriedly, hurry-scurry, impetuously, impulsively,
   like crazy, like mad, madly, overeagerly, overenthusiastically,
   overzealously, precipitantly, precipitately, precipitously,
   ramble-scramble, rashly, recklessly, slam-bang, slap-bang,
   slapdash, suddenly, wantonly, wildly