When resizing the Thunderbird window, it’s size is not persistent across re-boots when using Wayland and KDE 6.1 (but works fine if X11). I’ve tried both Kubuntu and Manjaro.
I’ve also tried 3 versions of Thunderbird (115.x, beta, and flatpak) in Kubuntu, all with the same problem.
Also tried “MOZ_ENABLE_WAYLAND=1 thunderbird” in Kubuntu, and nope.
I’m not sure what the core problem is, but I created a window rule for thunderbird with the property “size” and “remember”, and all is good now.