By making SessionStore accept :setupSyncHandler and :update messages from <xul:browser>s without a tab assigned we can preload a <xul:browser> in the background and assign a tab later. SessionStore will have the correct sync handler and know about the current content loaded in that browser. If the browser will never be assigned to a tab the received data will simply be discarded when the browser goes away due to the use of WeakMaps in SessionStore.
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