Does Flash Player work on Android 5 with Firefox 48
Adobe does not support Flash on Android beyond 4.0 but that does not mean that it won't work. The last available version is 11.1 which is not on Google Play so must be sideloaded from the Flash Player Archive.

Flash support in Firefox is being phased out and Chrome also. Other browsers, like Dolphin, continue to support Flash and work with the plugin installed from the archive.