Because many sites intentionally give you different code because you’re not in a chrome browser and that code is frequently tested with lower priority or not at all due to market share. And Firefox is able to run chrome code.
Additionally some sites actively tell you that your browser is not supported and downgrade the experience because you are not using Chrome.
But in reality the sites really only react to the user-agent, so doing this just makes them use the chrome code.
The social players all moved over to FFXIV