What alternative did you configure?
I do not set ICEWEASEL_DSP anywhere, so iceweasel uses the default value
of "none". Did you configure anything for this environmental variable?
Check it with "env | grep ICE".
Where may I change this configuration globally?
Just in /etc/profile specifically for iceweasel
or somewhere (KDE Control Center?) for all apps?
I would set it both in your shell's profile/rc-file and in
~/.kde/env/env.sh, see "man startkde". (The setting in the shell
profile/rc will not work if you start iceweasel from a desktop menu or
by clicking an icon.) Make sure you use "export ICEWEASEL_DSP=none".
I just found