Yes, all you need to do is to ensure that your browser is properly configured to load applets.
Logically, installing a Java Runtime Environment should setup your Firefox Java plugin.
Maybe this plugin is disabled in your Firefox, check it in Tools > Add-ons (and then Plugins tab).
If you are running on Windows, you should also check your Java settings in the Control Panel.
You also have another option, which is to try use another browser like Chrome or Safari.
Yes, all you need to do is to ensure that your browser is properly configured to load applets. Logically, installing a Java Runtime Environment should setup your Firefox Java plugin. Maybe this plugin is disabled in your Firefox, check it in Tools > Add-ons (and then Plugins tab). If you are running on Windows, you should also check your Java settings in the Control Panel.
You also have another option, which is to try use another browser like Chrome or Safari.