These funny blocks that look like parts of a picture are indeed parts of the header image. Looks like the margin and padding highlight somehow displays the wrong stuff. Just as a reference, the above usually looks like this:
I'm sure this Firebug bug :) gets fixed pretty fast but it made me remember that Opera contains a similar development tool: Dragonfly. It's not as hot and pretty as Firebug but the basic functionality looks pretty solid.
After this experience I will fire up Opera a bit more often when doing design stuff (right now I use Opera only for browsing). For the usual grunt work Firebug is absolute invaluable (alone the JS debugging …) but it's good to know that there is an alternative at least for some situations. I like variety.