When I started to work on that bug I noticed that we actually instantiate a context observer per domain (right now for Page and Runtime), which means we would register a lot of event listeners and observer notifications. That can slow down the browser and causes extra memory, and not speaking about all the duplicated internal events, which get send around. Instead we should make it a singleton per session.
Bug 1593226 Comment 13 Edit History
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When I started to work on that bug I noticed that we actually instantiate a context observer per domain (right now for Page and Runtime), which means we would register a lot of event listeners and observer notifications. That can slow down the browser and causes extra memory, and not speaking about all the duplicated internal events, which get send around. Instead we should make it a singleton per target.