Here's a screencast of the bug reproducing on a freshly reimaged Pixel 4 (with Android 13, essentially-untouched install of Nightly, Smooth Display enabled). In this case, the Firefox web-content area turns transparent *before* it reaches a fully maximized size. Also: just as noted in comment 6, Firefox's toolbar still seems to remain opaque; it's just the web content area that becomes transparent.
Bug 1899648 Comment 7 Edit History
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Here's a screencast of the bug reproducing on a freshly reimaged Pixel 4 (with Android 13, essentially-untouched install of Nightly, and Smooth Display enabled in system settings.) In this case, the Firefox web-content area turns transparent *before* it reaches a fully maximized size. Also: just as noted in comment 6, Firefox's toolbar still seems to remain opaque; it's just the web content area that becomes transparent.