In an ideal world we'd pass in the `privateBrowsing` flag and use that to both find the correct AUMID and binary to compare against, but because this code runs off main thread, and finding AUMIDs requires prefs access - we must pass in the AUMID from elsewhere.
Given that, checking that the target of the shortcut is one of the two possible Firefox entry points, and ensuring it matches the requested AUMID, seems reasonable.
There's no technical reason why we couldn't pass in the `privateBrowsing` flag to use in determining which binary we're looking for -- but I don't think it really adds any real benefit, and passing both it an the AUMID makes the interface more confusing.
Differential Revision: https://phabricator.services.mozilla.com/D155008
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