Firefox 55 and later will prevent Flash content from being loaded from
ftp or any other URL schemes except
https. This change aims to improve security, because a different same-origin policy is applied to the
file protocol, and loading Flash content from other minor protocols is usually not well-tested.
Developers can still test local Flash files by flipping the hidden
plugins.http_https_only preference through
about:config, but you may rather want to set up a local web server by using XAMPP for example.