Goofy needs you to run Mac OS X 10.10 or higher. This is because it uses WKWebKit to display the chat window. This framework was introduced with Yosemite and this is why it is not possible to run Goofy on older versions of OS X. Sorry!
I first thought about launching Goofy on the Mac AppStore, but Apple rejected it. After explaining them a little more about Goofy they allowed it. In the mean time I already released it on my own. So now I would need to create two different versions for direct distribution and the AppStore. I also really like the fast way of shipping updated with Sparkle. But perhaps I will release a version on the AppStore later.
Goofy is free, because I can not guarentee it will work tomorrow. It uses the Facebook website in some hacky way and any time Facebook changes their website this may break.
You can mute conversations in the settings, but Goofy will still show you notifications and a dock badge for muted conversations. This is because I can not detect from the HTML of the page if a conversation is muted or not. If anyone has a good idea for this, please let me know.
Basically, yes. But I heard of some people having problems with it. What might help is temporarily switching to Safari as default browser (if you are using a different one) or signing out and in from Safari.
This is an error caused by sandboxing. Download the latest version from the website and you should be good to go. Sorry for that!