This page says Firefox 8.x is supported, but it didn't work for me either when I tried to load an exercise.
http://www.mathxl.com/BrowserCheck/SystemRequirements.aspx?bookId=91719&autoh=yes&centerwin=yes

Contact that web site to let then know that they have a problem with a supposedly supported browser.

They are checking for Firefox 2 till 7, so 8 and above fails.

From http://interactmath.com/common/browsersniffer_1.js

this.is_gecko = (agt.indexOf('gecko') != -1);
this.is_firefox = (agt.indexOf("firefox") != -1);

if (this.is_firefox)
 {
  this.browser_name = "Firefox";
 }
        
this.is_firefox7 = this.is_firefox && (agt.indexOf("firefox/7") != -1);
this.is_firefox6 = this.is_firefox && (agt.indexOf("firefox/6") != -1);
this.is_firefox5 = this.is_firefox && (agt.indexOf("firefox/5") != -1);
this.is_firefox4 = this.is_firefox && (agt.indexOf("firefox/4") != -1);
this.is_firefox3 = this.is_firefox && (agt.indexOf("firefox/3") != -1);
this.is_firefox2 = this.is_firefox && (agt.indexOf("firefox/2") != -1);
this.is_firefox2up = this.is_firefox2 || this.is_firefox3 || this.is_firefox4 || this.is_firefox5 || this.is_firefox6 || this.is_firefox7;