If the CSS is not applied to the page, this means the style sheets could not be loaded, e.g. due to a network issue like a wrong URL.
Though if the CSS is applied to the page, this is probably a bug in Firebug. Having said that, as
Dennis already stated in the other thread, Firebug soon won't work anymore, because of internal changes in Firefox, which would have required to recreate Firebug from scratch. Because of that, the Firebug Working Group
decided to give up the development and maintainance of the Firebug extension and rather improve the Developer Tools built into Firefox. To make the transition to them a bit easier, the DevTools got a Firebug theme, which is automatically activated once multi-process Firefox gets activated.
Sebastian