(clickable)
Mainad has a valid height and width, however it isn't shown like subad1/subad2. Which are in essence exactly the same! (just a different background image).
Firebug shows my div as greyed out for some weird reason. Replacing the contents of mainad with just some text doesn't solve the problem (problem isn't related to inner elements)..
What's going on?! :(
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;
}