Reading the XUL Documentation would have helped! Rated 3 out of 5 stars

Well, I read something below that sometimes only INCOMPLETE sites will be saved.

The reason is simple: (autosave.js, line 12)

gBrowser.addEventListener("DOMContentLoaded", function(aEvent){autosave.always(aEvent);}, false);

DOMContentLoaded means: ** DOM body **.
And that means: everything *BUT* graphics!!
No gifs, no jpgs, no pngs - NADA.
No wonder that sometimes half of the page is missing, because the routine already fired when the page was not yet finished loading.

However, being an "advanced-in-the-making" developer myself, I can confirm this is anything but trivial!