* Install
this extension (A yellow security alert bar may appear at the top of the browser when you click 'Install'. Click on the bar and opt to allow permission. I've never had any trouble with anything installed from here.)
* When installed, close and re-open Firefox.
* Go to the Tools menu.
* Go into the User Agent Switcher menu and select: 'Internet Explorer 6 (WindowsXP)'
The sites mentioned should then think Firefox is Internet Explorer and let you play the games.