It seems that the Fire.app has the path to the system mono hardcoded to /usr/local/mono. This works when mono is installed from a PKG file, but it does not work if mono is installed from homebrew. It would be nice if Fire.app allowed editing the paths manually.
That currently is the case yes, but we’ll fix that. Note that his path matters only for running mono apps. Not for Fire’s internal use of Mono.