Here's a screenshot to demonstrate that this isn't Firefox-specific; it just depends on how wide `<input size="40">` works out to be (which is font-dependent and hence varies subtly between browsers), and whether or not that happens to be wider than your phone's screen.
Bug 1908674 Comment 4 Edit History
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Here's a screenshot in Chrome to demonstrate that this isn't Firefox-specific; it just depends on how wide `<input size="40">` works out to be (which is font-dependent and hence varies subtly between browsers), and whether or not that happens to be wider than your phone's screen.