I also tested Chrome's own responsive design mode with a Firefox-for-Android UA string (the string from comment 1), and they match our "Actual results" here too. So there's no Firefox-specific behavior on our side here at all, other than the UA string itself.
Bug 1885086 Comment 2 Edit History
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I also tested Chrome's own responsive design mode with a Firefox-for-Android UA string (using the exact Firefox UA-string from comment 1), and Chrome matches our "Actual results" in that configuration (showing the toolbar at the top-right of the viewport). So there's no Firefox-specific behavior on our side here at all, other than the UA string itself.