I've been playing with int.d scripts and accidently deleted the 03firstboot file. I was wondering what scripting was involved in it on an IHO build.
That's right. It's used primarily on devices that require proprietary apps, and in such cases will install the apps during firstboot. Some even use secondboot, thirdboot, and even fourthboot scripts for advanced installation requirements (I believe they set a persist.sys.PROP to keep track of boots). On the OV, we only need proprietary libs and binaries, so we don't make use of the firstboot scripts.
There are tons of other ways you could use it. I played around with the idea of an elaborate April Fools joke that would use it to erase all evidence of the joke after first boot. Thought better of it when I was browsing through some of the "Help I Bricked My Phone" threads, some people are just too gullible.
If some intrepid script writer (perhaps you) wants to take some time to write the appropriate firstboot.sh, it could be used to initialize some of the *2sd functionality during the first boot up after flashing an updated ROM. I don't use any of the *2sd scripts, but I know lots of people do.