Comment 4 for bug 32891

XIM is enabled, so this is relevant.

gtk2 by default on all text input areas has certain options on the menu that
pops up when you right-click. These options allow one to select the current
input method, and enter certain special Unicode control characters as well.
mozilla has completely replaced the gtk2 menus with its own menu, removing this
functionality.

This means that it's impossible for users to change the current input method
while running mozilla. The only way to change the input method is to quit
mozilla and restart it with the GTK_IM_MODULE environment variable set to
something different.

In fact, many users might not realize that they can set the GTK_IM_MODULE
variable. (e.g., export GTK_IM_MODULE=xim for XIM) If they don't know this,
then they won't be able to use any input method at all. Perhaps it should be in
the release notes.