Fix #5244, use incognito: spanning in manifest (#5294)
Fix #5244, use incognito: spanning in manifest (#5294)

Soon Firefox will disable add-ons without this, or require people to opt-in
mozilla-services/screenshots,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/pageshot,mozilla-services/screenshots,mozilla-services/pageshot,mozilla-services/screenshots