Merci Ubuntu: https://wiki.ubuntu.com/ComposeKey#Conf … 2C_etc..29

Gtk (such as also FireFox) and Gnome applications use (on an Ubuntu system) usually SCIM as the input method, not XIM (X Input Method) that relies on the configurations from (X)Compose. Therefore, you will often find that the "a" and "o" macron characters can yield the orange/male ordinals "ª" and "º". To still make it work, Gtk applications need to be "told" to use XIM instead.

"One Off" Configuration

For a single application you can do it by just setting the environment variable GTK_IM_MODULE for the application to be launched to xim:

$ GTK_IM_MODULE=xim firefox

More permanently that can be done for a single shell session like this:

$ export GTK_IM_MODULE=xim

This can, of course, also be persisted for a single user in your ~/.bash_profile, or for all users in /etc/environment. This is, however, the less elegant way, the more elegant way follows below.