The Firefox 55 v has this change: "Made the Adobe Flash plugin click-to-activate by default and allowed only on http:// and https:// URL schemes. (This change will not be visible to all users immediately. For more information see the Firefox plugin roadmap)"
This mean that about:config->plugins.http_https_only is set "true" by default so, "file" protocol (started by FB) will block FlashPlayer plugin. The workaround is just change the parameter to "false". -- Sent from: http://apache-flex-users.2333346.n4.nabble.com/
