Using Mozilla Firefox 68.0.1 on Ubuntu 19.04 x86_64 on a Dell G3 3579.
Using keyboard layout International (AltGr dead keys)
. The keyboard works as expected in the terminal or gedit, but for some reason, on Firefox, it doesn't. Pressing AltGr + ' doesn't create a dead key but just produces a '
.
Terminal and Gedit are Gnome products but FireFox is not. You could try the Unity Desktop:
Although written for 17.10 and 18.04 the same installation techniques apply with 19.04.
For a different set of screenshots with Ubuntu 19.04 installation of Unity see:
The only other thing I can think of trying is a different version of Firefox but I'm not sure how successful this would be:
The simplest way seams to put the call
setxkbmap [lang]
, where lang = de, en etc. to/etc/profile
or any other configuration file, read for all users.