FireFox extension ot attach CSS to every Web page

I need a FireFox extension that will automatically block ads on some popular websites in my country. Ti achieve this, I need the extension to include a reference to a CSS file (residing on my server) on every visited page. The CSS file will contain the commands to hide advertisements, and the link to it will be supplied by me. Your task is to write an extension that attaches this CSS style to every Web page, preferably even before the page loads (in a similar way that user-defined styles are applied in the preferences). Also, I need the script to browse through the DOM structure of the page after it is loaded, and if a DIV element with ID from predefined list is found, add a 960x40 IFRAME after it with predefined address.