why not try mozilla with xft? you wont have that problem since it is using xft 2.
can put your
`xlsfonts | grep simsun` ?
i remember there is a flag '-p-', change that to '-c-'
the problem comes from ttmkfdir program, it wrote the wrong flag.
and xfs everytime call ttmkfdir to reset fonts.dir when it starts.
# ls /usr/fonts/zh_CN/
encodings.dir fonts.dir fonts.scale mingliu.ttc simsun.ttf tahomabd.ttf tahoma.ttf
make sure this font dir is not in the xfs search path (/etc/X11/fs/config)
but in /etc/fonts/fonts.conf path section.
and in /etc/X11/XF86Config FontPath section