4
It's possible the themes your using are not optimised for use with firfox, or are using depreciated values in the .css
Check the theme folders and make sure you have styleNN.css for firefox/mozilla type browsers. If your theme contains only one stylesheet (style.css) then it's possible it may not render properly in browsers other then IE. A fully compliant and valid .css sheet will render in all browsers, so some themes will only carry one .css file.
You can also get the
WebDeveloper Extention for Firefox, which will allow you to view every element of a rendered page in Firefox, including ID and Class details used in a sites stylesheet. This is a great way for troublshooting problematic themes. Once you know the Class and ID of an element on a page, you can then edit your .css to fix it.
Hope this helps some.