program a Firefox extension to download a page

I want to apply a regex to a page whose URL only my extension is going to know. How do I program a Firefox extension to download the page?

From stackoverflow Delirium tremens

• Found! "To load from file://, http://, chrome://, resource:// and other URLs directly, use XMLHttpRequest or nsIChannel (example)." - File I/O

  From Delirium tremens