first, "Edit As HTML" box should not be taller than developer tools -- bug 1330620 if that bug is addressed, the next logical step should be to make it so that inspector's left pane is automatically scrolled upon selecting "Edit As HTML" so that the edit box fits within the current view. this way the user does not need to adjust the outer scrollbar's position in order to be able to scroll up and down with the inner scrollbar. the user should still be able to use the outer scrollbar to scroll up and down such that the textarea is not fully within view -- i'm only suggesting automatic scrolling each time an edit textarea is initially opened.
I'm starting to wonder if we shouldn't just make the editor as tall and wide as the inspector DOM panel, so that it completely overlays the DOM tree, like Firebug used to do. I don't see a big reason for leaving parts of the tree visible while editing HTML.
i think that would be fine.