Using Firebug, I can see some differences between the DOM->CKEDITOR->env between Chrome and Firefox. For Firefox, it's not recognizing the browser correctly--it thinks it's a mobile device rather than a "gecko" browser, and is (therefore?) labeling it as incompatible.
lists compatible browsers. They claim to support all major Firefox releases starting from 3, and apparently only through release 5. However, you can't even get release 5 from mozilla.org--their standard release is now 8. Not sure how to handle this.