Because you have't update your Flash Player. It's out of date, and that's something YOU need to fix.
Update here.
Your explanation is inaccurate. I have the latest Flash Player, and Four Second Firestorm starts with an error message saying Flash Player 9 is needed. I have seen other old games with a similar error message (in fact, AG has another such game called Line Runner that requires Flash Player 8), and I think the problem is that certain old games don't have an ability to read 2 digits. So these are the options I see:
1) Downgrade to Flash Player 9 if possible. I'm not sure if this is possible, but if it is, it should enable games that read only 1 digit of Flash Player.
If Flash Player cannot be downgraded, then such games will be unplayable until many years later if Flash Player keeps adding new versions (a version 80 should make Line Runner playable, and a version 90 should make Four Second Firestorm Playable). However, this will not happen for a long time, so AG should delete those two games and any other games that can't read a second version of Flash Player.
As for Four Second Firestorm itself, while you can't play it, there are two similar games that are still playable. Those games are Four Second Frenzy and Four Second Fury.
Also, I don't think AG can fix Four Second Firestorm. The only thing they could do to it is to delete it.