Anyone who’s used the English language knows from direct experience that multiple spellings of many words exist. Such is the case with this set of words.

Firsthand (all one word) is an adjective meaning that the information was obtained from personal experience or an original source; to wit: Thanks to the outdoor navigation course, we had firsthand experience using a compass. In this sentence, firsthand shows the kind of experience (which is a noun) that one had.

Firsthand also can be an adverb, as in: We’d seen firsthand the horrors of living in a war-ravaged country. In this case, firsthand shows how something was seen (which is a verb).

This is a North American spelling; in Britain, first-hand usually is used.

First hand (two words), however, is improper (no matter how well one knows English).