You actually had my same idea and thoughts (from bug 1523332) about UrlbarPrefs observer (even if I actually used a Set so registering multiple times the same observer is a no-op). I finally decided to remove it because there was only one consumer and there was risk of introducing windows leaks (in my case the observer was created in a window global object). But if you introduce an uninit() I may just go back using it.
Bug 1524950 Comment 2 Edit History
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You actually had my same idea and thoughts (from bug 1523332) about UrlbarPrefs observer (even if I actually used a Map so registering multiple times the same observer is a no-op and we don't invoke lots of observers for nothing). I finally decided to remove it because there was only one consumer and there was risk of introducing windows leaks (in my case the observer was created in a window global object). But if you introduce an uninit() I may just go back using it.