Silky smooth on both a two year old Snapdragon 8 Gen 1 (Xiaomi 12) using both Chrome and Firefox and on a iPhone Pro Max 15 using Safari. Might be some other problem with your phone?
Actually realized I was using Firefox Focus on that Snapdragon 8 Gen 1. Just downloaded Firefox to try it and it's also the same as Firefox Focus...no issue. Could it be an extension? Fwiw, I don't have any extensions installed on any of the browsers.
Strange that Firefox works fine for you on a slightly older version of the same platform.