My UI was actually on the top, and by changing settings it looks like it’s actually other way around. When UI is on the bottom the fixed element on page moves semi-smoothly (there is still a gap during animation), but when UI is on the top it is fucked up completely. When scrolling down the fixed element also scrolls down for a bit before appearing where it should be, and when scrolling up the reverse happens. So it looks like they didn’t account for that users actually use UI on the top and that it can affect fixed elements on the bottom of the page or it’s just bugged because they don’t test their stuff. I guess I need to send a bug report.

Edit: This bug seems to be at least 3 years old: https://github.com/mozilla-mobile/fenix/issues/16264

Edit 2: There’s actually a (I think) relevant issue on bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1797964