Html / Css - Div Element Hidden When It Shouldn't Be?

(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 sh

Solution 1:

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

Solution 2:

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;
}

Solution 3:

Firebug displays selectors without HTML content, or block element styles as gray. If you edit the HTML, and add some copy you will see the selector regain its colors.