Flash Player 2021 uses the Ruffle emulator, and Ruffle may not work for some Flash content yet. Please see https://ruffle.rs/#compatibility for more details on Ruffle's support for ActionScript 3.
Unfortunately, you will need to wait for Ruffle updates over the next few months/years. Ruffle is not affiliated with ModernKit.one, so we cannot provide a timeline for ActionScript 3 support.
We'll update the extension description and post-install page to make this clear.
