IE, any version of IE, doesn't like it when you stack CSS definitions (more than 1 value per definition) as this is a CSS3 ability which is only voluntarily included in most of the 'good' browsers and completely omitted by the rest (particularly MS who seems to be loathe to include anything in their browser architecture until it's a "standard" that must be followed).
My thoughts on this...
Problem 1: "<body class="home page page-id-21 page-template page-template-default">"
Problem 2: "<div class="skip-link screen-reader-text">"
and so on...
That being said, I'm no CSS expert but whenever I get extreme performance variation between IE and FF I look there first and after spending 5 minutes wading through your CSS stylesheet I got lost and decided it was time for a smoke :twisted:
Hope this helps somewhat :) Please remember to mark as solved once the issue is resolved.