Errors with different browsers

The two problems you noted isn’t related.

Error 404 basically refers to “Page Not Found”. It means that the link you’re trying to access isn’t available on your site. Re-check your links and make sure nothing’s broken. Or sometimes it’s just simply a server error.

It displays differently since browsers renders sites in a different manner (though most like you should get the Firefox the same and IE should follow because it’s more open to different code structures, even bad code structures actually).

It’s rather a big topic to discuss here but eHow wrote a good tutorial: https://www.ehow.com/how_7166758_make-display-same-ie-firefox.html