Thank you! I used Unity 2D to code the game and exported it to a website using WebGL (built into the Unity platform).
If you want to make a mod of FNF, it's more likely that your friends want to use HaxeFlixel to change the existing game. With mods, you'll have to download the Friday Night Funkin' game off of Github and make changes to that, before re-exporting it in order to publish it.