“Join the Conspiracy of Kindness” by Wade M on Flickr, CC BY-SA 2.0
I’ve seen them in the wild, and BuzzFeed’s Ryan Broderick recently rounded up a collection of them [content warning: profanity] — examples of firstable being used instead of first of all.
It’s #WorldKindnessDay today. So “firstable,” I’m going to refrain from ranting or ridiculing. I rarely rant … on my blog … and genuinely try to never ridicule. My aim, as always, is to kindly bring style and order to the world of writing. Even when it’s not World Kindness Day, I consider myself a kindness conspirator.
So “secondable,” I’m going to kindly point out that — much like the case of should of, would of, and could of (which I wrote about on Copyediting.com) — your ears are tricking you here. When spoken, -able and of all sound very similar, as do of and the contraction ‘ve. But they are not interchangeable. Here are the correct forms:
You could HAVE used first OF ALL and would HAVE used second OF ALL, if you had known. But really, you should HAVE simply gone with first and second.
Or, you could let your points stand on their own, without the unnecessary introductions. First points come first, second points come second, and people are generally smart enough to follow along. Really!
Simply say what you mean, without trying to give it a grand setup or overemphasize it. You will sound less defensive and, if it helps you avoid the unfortunate firstable error, better educated.
PurplePenning 
A bunch of us in GrammarGeeks are having at lot of fun with this one!
I saw that Ben Zimmer added it to the Eggcorn database, too. Which provides hours of browsing fun if you’ve never checked it out! http://eggcorns.lascribe.net/
In a spirit of kindness I would draw your attention to the unexceptionable split infinitive in: “My aim, as always, is to kindly bring style and order to the world of writing.” Not splitting would give “Kindly to bring style and order …” which has the effect of emphasising “kindly” by bringing it forward in the sentence, in the way that is similar to that used by Chorus in the Prologue to Henry V in the final line: “Gently to hear, kindly to judge, our play.”
Thanks, Michael! I appreciate the poetry, but prefer the “split” in this context.
Further reading on split infinitives:
— self-described snoot, Bryan Garner, has a 4-post series, beginning here: http://www.lawprose.org/blog/?p=1008
— professors/linguists/authors of English grammar, Rodney Huddleston and Geoffrey Pullum, have a short and clear post that ends with a roundup of Language Log posts: http://www.lel.ed.ac.uk/grammar/splitinf.html
— Stroppy Editor, Tom Freeman, took the time to check the history: http://stroppyeditor.wordpress.com/2013/10/28/to-helpfully-clarify-to-better-communicate-a-history-of-the-split-infinitive/
Bravo, PP. To thine own kind self be true. Your split has personality.