Wayland: Multi DPI Setup

Scaling on Wayland is handled by the application, in practice that means the toolkit (GTK, QT, etc.) (support status). GTK 3 apps (like GNOME Terminal) are fine, as you've found.

Firefox has come a long way, but menus don't position correctly yet (issue and related) - a development version is available here. Thunderbird would hopefully follow after Firefox. For Chromium work is underway, but it isn't finished either. Electron uses Chromium under the hood, so that can't be fixed until Chromium is done.

Because these apps don't fully support Wayland yet, they still use the X11 protocol with XWayland, which doesn't handle MultiDPI well (GNOME has ideas, but I doubt whether this will happen).

So I guess you'll have to wait until all applications support Wayland, but there are some workarounds that may be useful to some. Archlinux has some notes on HiDPI and Wayland, for example.