Firefox/Thunderbird 3rd level key not working on Wayland

Dear Community,

I have the following problem:

I am used to Apple’s keyboard layout w.r.t. the modifier keys. Therefore, I configured the keyboard in the following way:

Ctrl is mapped to Alt, Alt to Win
Key to choose the 3rd level: any win

This works perfectly in any QT-Application I encountered. However in Thunderbird and Firefox it does not. There my 3rd level modifiers have no effect at all. When I change the keyboard configuration in a way that Alt and the 3rd modifier aren’t on the same key the problem disappears. E.g. via

Ctrl position: Left Alt as Ctrl, Left Ctrl as Win
Key to choose the 3rd level: Right Win

This is surprising to me as the default keyboard layout also combines Alt (GR) and the 3rd modifier.

The second configuration is an acceptable workaround, however I would prefer a clean solution and I am also curious why this happens.

Also this only happens on Wayland. In x11 my original configuration works flawlessly.

Dit you encounter the same? Did you find a solution? Have you got any suggestions?