You can remove the bar for Firefox: Right click on the toolbar -> “Customize Toolbar…” (or Burger menu -> “More Tools” -> “Customize Toolbar…”), then in the bottom left uncheck “Title Bar” (This works in KDE, at least)

I’ve last seen it last month. And I have an old chat, where FluffyChat and (“old”) Element show all messages by now, but Element X can’t decrypt many and both Elements report that they can’t guarantee the authenticity of many messages (even my own). For a long time, my chat partner could only read messages I sent via FluffyChat but not those sent by Element. I have not checked if that is still the case.