[kubuntu] Inconsistent behaviour of Keyboard US, Alt Intl

Hi,
I have had this issue for quite a while actually on Kubuntu. Dead keys do not behave consistently across applications.
They would work in:
- Konsole (terminal)
- Dolphin
- Kate
- LibreOffice
but not in:
- chromium, atom
- firefox, thunderbird
- emacs (although I suspect emacs has probably its own way to handle alteative inputs).

It is probably a complex matter as the subject pops up on regular basis and could involve
OS handling of the input method as well as specifics of applications.

Any suggestion on how to make Firefox handle dead keys on Linux ?

Best

Trad