For HTML you even don't need an extension. All you have to do is to press Ctrl+A while the page is focused. Right-click the selection and choose View Selection Source. Then you'll see the HTML markup including all dynamic changes and you can save it via File > Save Page As...
In firebug FAQ:
Q: How can I save changes I made inside the CSS panel back to my source file?
A: This feature is not integrated into Firebug, but there are some extensions, which do this (or at least let you save the generated CSS):
1-cssUpdater (http://www.cssupdater.com/)
2-FireFile (https://addons.mozilla.org/firefox/addon/52365/)
3-Firediff (http://www.incaseofstairs.com/firediff/)