HTML / CSS - DIV Element hidden when it shouldn't be?

Do you have AdBlock installed? That might be hiding that div.

An extra little note: FireBug will actually show something like this in the Style pane when AdBlock Plus is blocking it (wrapped for clarity):

.mainad, #mainad {             undefined (line 233)
    -moz-binding:url(chrome://global/content/bindings/
     general.xml?abphit:707307897865#basecontrol) !important;
}