First is an adverb in this sentence (it modifies the "I do").
Adverbs can be somewhat flexible where they appear in a clause - before or after the words they modify - if the thing that it's modifiying is a verbal. In text, words that come first in sentence typically have emphasis, whereas in speech, it's how it is said that controls the emphasis. But there is not a great difference in meaning.
The topic was debated hotly between them.
The topic was hotly debated between them.
He ran away quickly. (This has emphasis on "ran away" over "quickly" - unless in speech it is said "He ran away quickly.")
He quickly ran away.
He ran quickly away.
Quickly, he ran away.
He doesn't want to go now.
He doesn't now want to go
Note that this doesn't work for adjectives and as always with English there's exceptions to everything (I can't think of any right now but I'm sure there are.)
Her touch was gracefully pleasant.
Her touch was pleasant gracefully (wrong).