Your question reminded me of a plug-in for Illustrator called Scriptographer:
http://scriptographer.org/scripts/
It's basically an Illustrator panel from which a user can run a lot of automated or interactive drawing scripts and tools that have been created by various developers.
I'm not sure if you're imagining drawing your fractal, tree-branch shapes by hand or scripting them somehow. Fireworks does allow you to run your own scripts, called Commands. However, I don't know if anyone's created any commands for Fireworks that could be useful in creating "fractal" shapes. Fireworks does have good vector drawing tools; on the other hand, it's intended primarily for web or screen use. Illustrator might be the better tool if you think you'd like to output your work for print. It also offers smooth previewing of your vectors at any zoom percentage, which is nice.
