Introduction
So, you're having trouble with the FFB client? This note is an attempt by me to write all the various steps you should consider once and for all in a single place to not have to say this over and over again. I strongly recommend going through each and every one of these steps.

This guide is slightly Windows-centric, but should be easily adaptable for MacOS and Linux users.

Basic setup

Get a clean Java installation
Download the latest version of OpenWebStart from openwebstart.com, and install.

It is also recommended to uninstall other versions of OpenWebStart, IcedTea, or Java you may have on your system unless you know you need them.

Associate "JNLP" files with Java Web Start
Go to the Games Page and click to spectate any of the games. Your browser will download a JNLP file to your downloads folder.

This file needs to be opened with OpenWebStart. Depending on your system, there are different ways of doing this.

In a modern Windows installation, right-clicking the file and selecting "Open With" followed by "Launcher for .jnlp applications according to JSR-56":



If this doesn't show up as an option in a submenu, you can browse to the location of the javaws.exe program in the OpenWebStart installation directory. By default, this is in: C:\Users\[your user name]\AppData\Local\Programs\OpenWebStart
Firefox specific settings
If you are using Firefox, it uses its own setting for how to open downloaded files. In the settings, find the Applications section and set up JNLP to be opened with the Launcher as above:



Uninstall 3rd party anti-virus or "Internet protection" software
Symantec, F-Secure, AVG, etc etc. It's all a bad idea for various technical reasons (at some point, I'll write a longer thing about this). The short of it is that you're better off without them. Uninstall, and re-enable Windows Defender.

Java Configuration

OpenWebStart, as opposed to the legacy Oracle Java 8 has default settings which are mostly good. There is an "OpenWebStart Settings" application where you can check your settings. The most important setting is in the Logging panel, where you can make the log for the client show. If you need to do this, these are the settings you want to have:



You can also verify the "Proxy Settings" and change it to "No Proxy" as that is very unlikely to be used and there's no need to spend time trying to detect it.

In the section called "JVM Manager", you can manage the installed versions of Java. This is mostly a thing if you need to remove old versions, as OpenWebStart will automatically download the appropriate version of Java you try to run Java programs.