Well, since first in there is an adverb and adverbs are a lot more flexible than other parts of speech in English in that they can sometimes freely float around your sentences without affecting their meaning, why can't it be positioned in the middle of a sentence? What would be wrong with doing that? The emphasis might slightly get shifted, but that's okay so long as you know which part of your sentence you want to receive more stress. The following two examples semantically are exactly the same. The only difference is in the placement of first:
Help me first and then I'll help you.
First, help me and then I'll help you.
I, personally, can hardly pinpoint the difference between these two sentences apart from the fact that in the first sentence first is positioned in the middle of the sentence and in the second sentence it's at the beginning of the sentence.
But not all adverbs as flexible as this one, though. Adverbs like quite, almost, hard and many others usually don't have that kind of flexibility as adverbs like fist do. Meaning, their position in the sentence is more or less fixed. For example, I don't think that it's at all possible to make quite change its place in this sentence without completely breaking it or significantly changing its meaning:
That doesn't sound quite right.
That doesn't sound right quite. [sounds wrong]
That doesn't quite sound right. [fine, but I think the first sentence sounds better]
Quite that doesn't sound right. [sounds wrong]