I can't recall ever testing out whether that version of Firefox [52.5.3 ESR] would actually open/play .swf files.
In theory it should, because the functionality change occurred in v 55, but since the ESRs are updated for security tweaks, one of which was the change, maybe, maybe not.
But it's a thought to keep in mind in the event of an eventuality.
The new Great Equalizer is the SEND button.
In Memory of Harv: Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
