CSS & Validator Links

I am having a problem getting a CSS defined background to a CSS element #header to show in Firefox. I did a search for the problem and found a pretty good thread about the problem at Webmaster world. The general consensus was that IE is non-standards compliant, and Firefox is. So one should write for Firefox and it'll all render in IE. Below are a couple-a links to validators.
CSS Validator
HTML Validator

I just found a link specific to background images and how to make them appear:
Click here to view