One tool that might help is the CSS browser in Firebug. It will show you exactly where the computed styles came from. I find it indispensable for helping to find out "Why the #@(% is that blue?" (or in your case, black).
Author and all-around good cowpoke
Joined: Mar 22, 2000
It just got stranger...
I moved the files to another machine with the same (10.0) version of Firefox and they rendered correctly!
Both Foxes have just been updated so Fox-1 must have some kind of damage or configuration problem which prevents it from displaying HTML with certain CSS properties. Looking at the plugins etc I don't see any difference, guess I will uninstall Fox-1 and do a fresh install to see what happens next.