If you're modifying some content DOM from chrome code, you need to make
sure the child you're appending was created using the unprivileged
document, not the global document.
// This is chrome code, this is wrong because the span is now chrome and
you're trying to insert it into content
// This is right, appending a content node to another content node
I've hit this issue at least three times when upgrading stuff for Gecko
2.0. Might be what you're looking for :).