By default, Firefox requires you to manually click to enable website content that requires Flash. This improves your security and stability while browsing because you are less likely to be affected by Flash security bugs or crashes. See Why do I have to click to activate plugins? for details.
Adobe, Mozilla, and other browsers will end support for the Flash plugin in January 2021. Web pages that use Flash, such as some old Facebook games, might stop working or be blank. Those web pages will need to be redesigned to use HTML instead of Flash. There will be no setting to re-enable Flash. To learn more, see Mozilla's Plugin Roadmap for Firefox and the End of support for Adobe Flash article.