need a firefox extension through dll injection and xpcom (or whatever works)(repost)

I need a firefox extension/plugin/whatever made. This should NOT show up in the extension/plugin list in firefox and should work from versions 2.x through 4.x. This means that you liikely will NOT be able to use the traditional xul/javascript to make it. I recommend dll injection and using xpcom -- but whatever works -- it's up to you. I'd like to avoid using .net if at all possible as I don't want to have to download it if it's not there as this should support xp, vista, 2k, win7, etc. (no 98 support needed) The function of this will be to serve up ads -- you don't need to do this part -- all you need to do is inject a div containing some text taken from a url into a page if it's a google, yahoo, or bing search page. This ad should be taken from a website -- so post the keyword (or url as I can strip out the keyword server-side) and I'll hand you back the ad txt. This should be relatively simple for someone who has done this before. The user will know about this plug in and can easily uninstall it if wanted.