So I wanted to have editable XUL treecells for something, so I created a binding which seems to work. Well, except the text cursor doesn't appear, but one can still edit and select cell text.
Various Mozilla issues prevent it from working completely with custom views or RDF content (rendering bugs), or menulists in the cells (mouse events don't work). But it does work with remote XUL.
To edit a cell, double-click on it.