I don't know how it's generating shadows, but if it's via the CSS2.1 specification (or is it CSS3?) Firefox has no plans to support it until Firefox 3.next. There's a bit of a long complicated reason for the delay, but it originally wasn't part of the specifications to my understanding.
It has always supported background colours, the code just must have been badly written.
Firefox messes up alot of things
Give me a single example where Firefox genuinely messes things up and the author hasn't just badly written the website (and I count ActiveX as badly writing a website
)