Bug 1086958: move new notification bar implementation of the default browser prompt behind a pref, revert to old modal dialog implementation, r=dao

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extra : rebase_source : 85d09b7b6e25d84aa3ba5615aba7f09f1c2f6394
Bug 1091901: fix tracking protection UI icon for hidpi, r=gijs

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extra : rebase_source : d8c1e94fa80a3bed1f9ecb0edb93166b04dc2112
Bug 1092808: fix issue with polaris pref observer being registered too early (causing polaris prefs to be reset on restart), r=dolske

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extra : rebase_source : 5c87b94f621f3bc3c91c9fd918e58de6676c7eb2