Firstable is kind of a word now.
It is an eggcorn of 'first of all', and one that appears to have arisen independently among English language learners, teenagers, and other English Innovators with some regularity over the past few decades. It is just amusing enough to be echoed back and forth across Tumblr, Twitter, Buzzfeed, and other meme hosts; first someone uses it, then someone mocks them cruelly, and then others use it ironically/humorously, and then people mock them cruelly, and the cycle continues.
Firstable is used just as is 'first of all' when framing arguments. It may be accompanied by the words secondable and thirdable, which, aside from the benefit of being of clear and regular in grammatical form, avoid the whole first/firstly and second of all grammar peeves, replacing them with a single, united peeve.
While haters do hate, I like this word alot, and use it irregardlessly.