Update the following content into these 3 files then you will be able to switch to scim input in firefox.
/etc/X11/xinit/xinput.d/scim
/etc/X11/xinit/xinput.d/none
/etc/X11/xinit/xinput.d/default
#
# Use “X input Method” for all applications
#
# Per Ming’s Documentation in SCIM, XIM Input Method is activated
# not only for old X-applications but also for GTK and QT application.
#
# If a user wish to use, GTK Input Method, (s)he can right-click input
# area and select “Input Methods” and change from “X input Method” to
# “SCIM Input Method”.
#
XIM=SCIM
XIM_PROGRAM=/usr/bin/scim
XIM_ARGS=”-d”
XIM_PROGRAM_SETS_ITSELF_AS_DAEMON=yes
GTK_IM_MODULE=”scim-bridge”
QT_IM_MODULE=”scim-bridge”
DEPENDS=”scim,scim-anthyscim-cannascim-chewingscim-pinyinscim-hanglescim-primescim-skkscim-tables-additionalscim-m17nscim-uimscim-tables-jascim-tables-koscim-tables-zh”
Reference: http://ubuntuforums.org/archive/index.php/t-528382.html
