Hi

I have had a look and it looks like (reading the MSN.com support site) that MSN Games is designed to only run in Microsoft's browser.

If you are looking for a fun game you can play in Firefox, I can recommend BrowserQuest.

There are many great game sites out there.

I don't know if this will help;

Fire IE (FireIE, Enhanced IE) {web link} Embeds Internet Explorer(IE) to use the powerful Firefox add-ons. Not only an enhanced IE Tab, but also an enhanced Internet Explorer with Adblock Plus and FireGestures support. Switch to the IE engine in one click and dismiss your IE.