Currently, debian sid is receiving a lot of new software and that’s why various things are broken. Iceweasel 3.5.19 with the default installed theme lacks of the backward and forward icons. The trick by using an alternate theme from the addons page was only working until a reboot. formorer introduced a better working way on that:
$ sudo apt-get install tango-icon-theme $ echo 'gtk-fallback-icon-theme = "Tango"' >> ~/.gtkrc-2.0