Firefox extension that overlays persistent iFrame?

Is it possible to build a Firefox extension that displays a floating, persistent iFrame over the page content?

I know it's possible to add iFrames using XUL. For instance, you can add an iFrame to a persistent sidebar. However, I want the iFrame to f开发者_StackOverflow中文版loat over the page content, not cause the content to shrink. So far, my only option is to add the iFrame to the DOM, then use CSS "fixed" positioning to float the iFrame.

The iFrame must also persist across page loads, exactly as the sidebar does. Adding an iFrame to the DOM, unfortunately, causes the iFrame to vanish when the browser renders a new page (e.g., after clicking a link).

Any clues?

Thanks!

Another add-on you can look at is Shopping Helper It has an iframe at the bottom whenever the page is displayed a product

Yes. I was able to do this by setting the attribute noautohide in the panel you use.

E.g.

<panel id="yourOverlay" noautohide="true">

You might be able to do something like this with Greasemonkey, it allows you to customize how web pages look and you could make your script available to others.