I had some style made up of two names, such as "Blue Swirly". I found out that MSIE was displaying this style properly but Firefox was not.
The problem was tracked down to the modification to the index.php file.
What the WP Style Switcher outputs is:
Note the lack of quotes around the filename. If you have a space in your CSS filename, then you end up with something like:
@import url(/path/to/my style.css);
At this point, IE assumes that the text between the brackets is a single URL, and Firefox assumes you're passing two commands.
If you want to fix it without needing to rename your styles, you can simply add quotes around the style filename in your index.php:
@import url( "<?php wp_stylesheet(); ?>" );
@import url( <?php wp_stylesheet(); ?> );
The fellow gamer that tracked this down (alas, it was not me) has put up a test install of WordPress that does this at http://www.totalbiscuit.com/wordpress/ . I've since fixed it at my site on Chasing the Wind and can use "Blue Swirly" again.
Try the "blue fade" style.
Hope this helps somebody.