In Firebug’s HTML tab, right-click the root node and select “copy HTML”. Then paste to a text editor.
Without Firefox Add-Ons, you could use a bookmarklet like this:
javascript: var win = window.open(); win.document.write('<html><head><title>Generated HTML of ' + location.href + '</title></head><pre>' + document.documentElement.innerHTML.replace(/&/g, '&').replace(/</g, '<') + '</pre></html>'); win.document.close(); void 0;