This is a bug on old Android 9 or 10, with recent Firefox.
Coincidentally, they have a simultaneous similar bug (but less harmful) on recent iOS!
Same kind of websites (including Discourse) is breaking design, triggered by the keyboard placeholder.
Here is a giant top padding on the bottom keyboard placeholder, plus small top bar, while over there is a small bottom margin on the bottom bar.
So I thought maybe Firefox uses WebKit, like iOS, but no, it doesn’t seem so.