They just released a new version that fixed this issue - at least for me

Right- click on menubar, select “customize toolbar”, select " titlebar" on left bottom. See here: https://www.reddit.com/r/kde/comments/udvxje/why_are_the_window_decorations_for_firefox/

Otterly specific setup

The first (last) column makes it look like a Trump style haircut

To be fair, this screenshot also does not have the default adwaita icon pack selected but instead something what I think might be the Mint theme(?)

It says here that LibreTube works, and I just checked and can confirm. In its settings, choose SponsorBlock -> Enable DeArrow.