Two Qs (Index display and Floats)

1.) I searched around for it but I couldn’t find it, is there a basic PHP command I can use within my index that will display something only if it’s the main index? EG, I want to wrap a certain DIV in PHP so it’s only outputted if WordPress says it’s the main index. Can’t figure this out on my own.
2.) I hate to ask a CSS/HTML question, but this is killing me and looks completely like a Firefox error to me. I’m trying to float an audio posts box on my page ( https://ihavebeenfloated.org ) and it works fine in IE, and it works fine in Firefox when I’m looking at my index page on my computer, without running it through WP, but one it’s out on the web, Firefox is blocking out the elements appropriately but the div itself disappears completely. So absolutely positively confusing.
The CSS is at https://ihavebeenfloated.org/superstyle.css … The z-index:666; is what got it to start displaying in Firefox on my local copy.