See https://bugzilla.mozilla.org/show_bug.cgi?id=1476030#c15. We are porting over some behavior from document.persist into xulStore.persist that is a bit inconsistent. It will remove a value from the store, but only if it was already set with a non empty value. So: ``` node.setAttribute("foo", "bar"); document.persist(node.id, "foo"); node.removeAttribute("foo"); document.persist(node.id, "foo"); ``` Will ultimately remove "foo" from the store, but: ``` node.removeAttribute("foo"); document.persist(node.id, "foo"); ``` Would write "foo" into the store with an empty value. This leads to code like https://dxr.mozilla.org/mozilla-central/rev/085cdfb90903d4985f0de1dc7786522d9fb45596/browser/base/content/browser-sidebar.js#81-85: ``` if (this._box.hasAttribute("positionend")) { xulStore.persist(this._box, "positionend"); } else { xulStore.removeValue(document.documentURI, "sidebar-box", "positionend"); } ``` Which could be rewritten (if we adopt this proposed change) to: ``` xulStore.persist(this._box, "positionend"); ``` We'll need to audit current callers to xulStore.persist to ensure they aren't relying on the current behavior.