Attempted to reproduce on 85.0.2, 74.0, 85.0 with Kubuntu 19. No issues on my part with default or while being logged into the Youtube/Firefox account(s). One more thing that could be checked by Teimuraz Abashidze could be an attempt to log into Youtube on the official build as well even though it's unlikely.
Bug 1693349 Comment 7 Edit History
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Attempted to reproduce on 85.0.2, 74.0, 85.0 with Kubuntu 19(VM). No issues on my part with default or while being logged into the Youtube/Firefox account(s). One more thing that could be checked by Teimuraz Abashidze could be an attempt to log into Youtube on the official build as well even though it's unlikely.