It looks like an issue with how LXDE reads the IceWeasel icons specifically:
https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=803165
There's a workaround describe on that bug page - regenerating the PNG icons from an SVG original, using inkscape in command-line mode.
for n in 16 32 48; do inkscape -z -e /usr/share/iceweasel/browser/chrome/icons/default/default$n.png -w $n -h $n /usr/share/icons/hicolor/scalable/apps/iceweasel.svg; done for n in 16 32 48 64 128; do inkscape -z -e /usr/share/icons/hicolor/$x$/apps/iceweasel.png -w $n -h $n /usr/share/icons/hicolor/scalable/apps/iceweasel.svg; done