Instead of calling `document.persist(id, attrName)`, please call `Services.xulStore.persist(node, attrName)` instead. The new function was added and all existing callers were migrated in Bug 1476030. I'm planning to remove document.persist from XULDocument in Bug 1476678.
This change helps reduce the differences between XULDocument and HTMLDocument, which is important for top level HTML window support (Bug 1453783).
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