The issue is verified fixed using Fx67.0b14 buildID: 20190423190209 (intermediary build). The user receives a message in the about:profiles page to restart the browser when another dedicated install creates a dedicated profile.
Bug 1529879 Comment 21 Edit History
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The issue is verified fixed using Fx67.0b14 buildID: 20190423190209 (intermediary build) on Ubuntu 18.04, macOS 10.14 and Windows 10 x64. The user receives a message in the about:profiles page to restart the browser when another dedicated install creates a dedicated profile.