The file HostServicesFactory.java (I cannot find it in the JFX8+ repo) contains this line of code, for Linux. It should be re-arranged and extended for better compatibility:
- static final String[] browsers = new String[]{"google-chrome", "firefox", "opera", "konqueror", "mozilla"}; + static final String[] browsers = new String[]{"xdg-open", "gnome-open", "google-chrome", "chromium", "firefox", "opera", "konqueror", "mozilla"};