Just to rule out the unlikely-but-possible: does it still happen if, instead of `--allow-downgrade`, you start Firefox with a dummy parameter like `--beware-the-jabberwock` instead? (If so, it's indicative of a race condition rather than something `- -allow-downgrade` deterministically averts, and will probably fail occasionally.)
Bug 1824110 Comment 7 Edit History
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Just to rule out the unlikely-but-possible: does it still happen if, instead of `--allow-downgrade`, you start Firefox with a dummy parameter like `--beware-the-jabberwock` instead? (If so, it's indicative of a race condition rather than something `--allow-downgrade` deterministically averts, and will probably fail occasionally.)