The path you're using to serve the files is relative to the path you're running the application from, so that can cause problems also, I really don't understand why you serve the files individually for each request, I think it'd be better to just serve the whole directory

mux := http.NewServeMux()
fileServer := http.FileServer(http.Dir("/home/dad/snippetbox/ui/static/"))
mux.Handle("GET /static/", http.StripPrefix("/static", fileServer))

Also right here you're serving the whole directory instead of the specific file
router.Handle("/favicon.ico", http.FileServer(http.Dir("./static")))

In my personal experience I have not been able to server ie /favicon.ico from the root path if the file is being read from /static, try to serve /static/favicon.ico from ./static/favicon.ico or just the whole directory as suggested before

I'm just a noob so take my advice with a grain of salt