如何使用Firefox扩展注入Javascript

I am building a Firefox extension and have come a long way. One of the parts of my extension is where I display a DIV at the bottom of the page and I want to put an ad into the div, but for some reason every time it does so, it redirects to another page entirely with the ad on it. So it looks like its not allowing me to insert this code below into the div that sits at the bottom. Any chance anyone can help?I am building a Firefox extension and have come