EDIT: I found the website that was offering fireworks brushes again!
Since version 1.2, FireAlpaca supports Brush scripts, which, after doing some research, turns out to be "mini programs" written in the Lua programming language (more on brush script programming) . FireAlpaca comes with some brush scripts, but for those of you who want more of these, I have here a couple of Japanese websites/blogs where you can download more brush scripts. Some, if not most of the brushes only have only Japanese menu though, so you might have to just guess and play with the brushes or get someone/thing to translate it for you.
http://p10memo.blogspot.jp/ <-- bristle brush, nice pen brush, and manga effect brush.
Found the website under a new URLTHIS SITE IS OFFLINE! Sorry about that ====================