(1) `loadPrefs` was called with string instead of boolean
(2) Sense of boolean `popup.breadcrumbs.usePath` was inverted
(3) `loadPrefs` wasn't passing default to `getPrefs`
(4) `JSON.stringify(string)` added quotes to string
(5) Preferences `manage.enabledFirst`, `observer.observeFrameContent`, `observer.observeFrameLoad` aren't used