Since bug 564432 got fixed we now carry a duplication of the storage for the children in the DOM. This extra storage costs memory, and more importantly, it costs time and code complexity to maintain both ways of storing child nodes. The storage model we want long term is what was implemented in bug 564432 (first child and next/previous sibling), so the array of children is not necessary any more. But we do still have consumers of the old storage, and we need to find those and convert the remaining ones to not rely on the index of the child etc or fast access to the n:th child etc. This bug is about removing the remaining code that depends on the array storage, and also removing the array storage.

I believe, this is a duplicate of bug 651120.