I have the same issue:
RichFaces 4.2.0 and 4.2.3
no matter if using IE or Firefox.
Reloading page makes the problem more evil in most cases.
After closing Browser and reopen the page it works again.
Tyr to install Firebug in Mozilla and try to load and test your css is loading in the page load or not.Try adding it in the mainpage or template page if you have in your application means.So in that case no need to load css in all the pages of JSP or XHTML
Also share some page and code.