Or rather, why wasn't CFSelenium picking up the Firefox directory I specified in the path environment variable?
I knew it was due to caching but I wanted to make sure the JVM was indeed caching the path variable.
path = createObject("java", "java.lang.System").getenv("PATH"); dump(path.listToArray(";"));
This proved the change I has made wasn't being picked up.
I couldn't find a way to refresh this so I ended up reboot my machine which meant the changed were picked up.