Ah, thanks. I didn't know that about the semicolon in general. Unfortunately that didn't fix the problem, though. Even when I put #000000 in directly it still shows up as white in IE and black in Firefox.
Edit to add:
The problem is Firefox doesn't handle css in <hr> very well, as described
here. So I have to just use color="white" instead of the style tag.
Thanks for your help, though.