I figured out how to input Esperanto letters in Mozillish browsers. It’s because Mozillish browsers use GTK+ for user interface, and GTK+ is lame. I have to set an environment variable telling GTK+ not to use any of its lame input methods and fall back to the default X windows stuff.
This ought to get rid of the need to manually tell gnome-terminal the same thing. I was too lazy to figure out how to get gnome-terminal to do this automatically.
Opera had no similar problems, because it uses Qt instead of GTK+. Qt also is lame, but it's relatively good with languages.