Just a quick note, Ad Muncher is browser independent, it sits out in front of the browser, so whatever you do with it, it will work the same with whatever browser you use.
As for Firefox not working properly with the site, but MSIE working fine, by design, Firefox does not display web pages exactly the way Internet Explorer does. On the one hand, there are W3C standards and on the other hand, the way MSIE works. They are different. Firefox and Mozilla, Opera and others have opted to obey the W3C standards so page rendering differences between all these browsers and IE are to be expected.
Certain sloppiness in some HTML coding that IE lets slip by, Firefox won't put up with it. Certain MS API's and javascripts do things in their own special way, a way that MSIE is fine with, but not so much with browsers that adhere to the standards. There are some java calls on the Panther Web site, for example, that are particularly offensive to the standards, and Firefox won't have anything to do with it. To make matters worse, all too often the popularity of Internet Explorer and the laziness of some web site designers combine to result in many web sites that are only tested against IE. The Web designer says, "Looks good, tastes great, less filling. I'm done."