Every time I play games, I see that a plugin is required, but I have already installed "flash" from software center. And also, I am able to see youtube videos but still this is only happening in firefox.

Every time I play games, I see that a plugin is required, but I have already installed "flash" from software center. And also, I am able to see youtube videos but still this is only happening in firefox.

You must restart Firefox to load installed plugins. The same for Chrome/ium. Plugins are loaded at startup time.