XCompose and Firefox, GTK applications.

[purpleposeidon]

Executive summary: export GTK_IM_MODULE=xim

Should go live in the README.

https://wiki.ubuntu.com/ComposeKey#Configuration_for_Gtk_Applications_.28Gnome.2C_FireFox.2C_etc..29

[bodograumann]

Also, it seems the include directive is not supported in GTK :-(
Compare https://gitlab.gnome.org/GNOME/gtk/blob/master/gtk/gtkcomposetable.c#L243