Chrome has quite a few components that are written in HTML/JS, but not the UI itself (i.e. the tabs and address bar). In Firefox, everything above Gecko is written in XUL and JS. A cool way to demonstrate this is to navigate (in Firefox) to chrome://browser/content/browser.xul. You will see a new browser window inside of your browser window.

It's incredibly satisfying to do that repeatedly, nesting Firefox inside Firefox inside Firefox and so on until you can't paste a URL in anymore.

Interestingly, though, the new menu introduced with Australis doesn't appear to be functional unless it's in the top-level window; the menu buttons in nested windows don't seem to do anything.

Could XUL/JS be used to customize FF for an enhanced view of "related pages"? E.g. the user interface is separated into 2 portrait-sized frames. One XUL frame shows the destination web page. The other shows related information, which could be links, images, thumbnails of other pages, etc. Does something like this exist?

You can do whatever you want from an XUL perspective, but the split-pane behaviour you describe already exists (sidebars). I don't know if there are any extensions that provide a sidebar like that, though, but it's certainly possible.

Good to know. I've only seen the history sidebar that is an outliner tree of links, didn't realize full HTML/JS was available. If a sidebar can respond to pixel-level events in the main window, many interesting augmentation apps are possible.